diff --git a/pkg/kv/kvserver/allocator/allocatorimpl/allocator.go b/pkg/kv/kvserver/allocator/allocatorimpl/allocator.go
index dda37590ec81..5210fa9ef308 100644
--- a/pkg/kv/kvserver/allocator/allocatorimpl/allocator.go
+++ b/pkg/kv/kvserver/allocator/allocatorimpl/allocator.go
@@ -133,6 +133,73 @@ const (
 	AllocatorFinalizeAtomicReplicationChange
 )
 
+// Add indicates an action adding a replica.
+func (a AllocatorAction) Add() bool {
+	return a == AllocatorAddVoter || a == AllocatorAddNonVoter
+}
+
+// Replace indicates an action replacing a dead or decommissioning replica.
+func (a AllocatorAction) Replace() bool {
+	return a == AllocatorReplaceDeadVoter ||
+		a == AllocatorReplaceDeadNonVoter ||
+		a == AllocatorReplaceDecommissioningVoter ||
+		a == AllocatorReplaceDecommissioningNonVoter
+}
+
+// Remove indicates an action removing a replica, i.e. in overreplication cases.
+func (a AllocatorAction) Remove() bool {
+	return a == AllocatorRemoveVoter ||
+		a == AllocatorRemoveNonVoter ||
+		a == AllocatorRemoveDeadVoter ||
+		a == AllocatorRemoveDeadNonVoter ||
+		a == AllocatorRemoveDecommissioningVoter ||
+		a == AllocatorRemoveDecommissioningNonVoter
+}
+
+// TargetReplicaType returns that the action is for a voter or non-voter replica.
+func (a AllocatorAction) TargetReplicaType() TargetReplicaType {
+	var t TargetReplicaType
+	if a == AllocatorRemoveVoter ||
+		a == AllocatorAddVoter ||
+		a == AllocatorReplaceDeadVoter ||
+		a == AllocatorRemoveDeadVoter ||
+		a == AllocatorReplaceDecommissioningVoter ||
+		a == AllocatorRemoveDecommissioningVoter {
+		t = VoterTarget
+	} else if a == AllocatorRemoveNonVoter ||
+		a == AllocatorAddNonVoter ||
+		a == AllocatorReplaceDeadNonVoter ||
+		a == AllocatorRemoveDeadNonVoter ||
+		a == AllocatorReplaceDecommissioningNonVoter ||
+		a == AllocatorRemoveDecommissioningNonVoter {
+		t = NonVoterTarget
+	}
+	return t
+}
+
+// ReplicaStatus returns that the action is due to a live, dead, or
+// decommissioning replica.
+func (a AllocatorAction) ReplicaStatus() ReplicaStatus {
+	var s ReplicaStatus
+	if a == AllocatorRemoveVoter ||
+		a == AllocatorRemoveNonVoter ||
+		a == AllocatorAddVoter ||
+		a == AllocatorAddNonVoter {
+		s = Alive
+	} else if a == AllocatorReplaceDeadVoter ||
+		a == AllocatorReplaceDeadNonVoter ||
+		a == AllocatorRemoveDeadVoter ||
+		a == AllocatorRemoveDeadNonVoter {
+		s = Dead
+	} else if a == AllocatorReplaceDecommissioningVoter ||
+		a == AllocatorReplaceDecommissioningNonVoter ||
+		a == AllocatorRemoveDecommissioningVoter ||
+		a == AllocatorRemoveDecommissioningNonVoter {
+		s = Decommissioning
+	}
+	return s
+}
+
 var allocatorActionNames = map[AllocatorAction]string{
 	AllocatorNoop:                            "noop",
 	AllocatorRemoveVoter:                     "remove voter",
@@ -268,6 +335,19 @@ func (t TargetReplicaType) String() string {
 	}
 }
 
+func (s ReplicaStatus) String() string {
+	switch s {
+	case Alive:
+		return "live"
+	case Dead:
+		return "dead"
+	case Decommissioning:
+		return "decommissioning"
+	default:
+		panic(fmt.Sprintf("unknown replicaStatus %d", s))
+	}
+}
+
 type transferDecision int
 
 const (
@@ -595,6 +675,146 @@ func GetNeededNonVoters(numVoters, zoneConfigNonVoterCount, clusterNodes int) in
 	return need
 }
 
+// WillHaveFragileQuorum determines, based on the number of existing voters,
+// incoming voters, and needed voters, if we will be upreplicating to a state
+// in which we don't have enough needed voters and yet will have a fragile quorum
+// due to an even number of voter replicas.
+func WillHaveFragileQuorum(
+	numExistingVoters, numNewVoters, zoneConfigVoterCount, clusterNodes int,
+) bool {
+	neededVoters := GetNeededVoters(int32(zoneConfigVoterCount), clusterNodes)
+	willHave := numExistingVoters + numNewVoters
+	// NB: If willHave >= neededVoters, then always allow up-replicating as that
+	// will be the case when up-replicating a range with a decommissioning
+	// replica.
+	return numNewVoters > 0 && willHave < neededVoters && willHave%2 == 0
+}
+
+// LiveAndDeadVoterAndNonVoterReplicas splits up the replica in the given range
+// descriptor by voters vs non-voters and live replicas vs dead replicas.
+func LiveAndDeadVoterAndNonVoterReplicas(
+	storePool storepool.AllocatorStorePool, desc *roachpb.RangeDescriptor,
+) (
+	voterReplicas, nonVoterReplicas, liveVoterReplicas, deadVoterReplicas, liveNonVoterReplicas, deadNonVoterReplicas []roachpb.ReplicaDescriptor,
+) {
+	voterReplicas = desc.Replicas().VoterDescriptors()
+	nonVoterReplicas = desc.Replicas().NonVoterDescriptors()
+	liveVoterReplicas, deadVoterReplicas = storePool.LiveAndDeadReplicas(
+		voterReplicas, true, /* includeSuspectAndDrainingStores */
+	)
+	liveNonVoterReplicas, deadNonVoterReplicas = storePool.LiveAndDeadReplicas(
+		nonVoterReplicas, true, /* includeSuspectAndDrainingStores */
+	)
+	return
+}
+
+// DetermineReplicaToReplaceAndFilter is used on add or replace allocator actions
+// to filter the set of live voter and non-voter replicas to use in determining
+// a new allocation target. It identifies a dead or decommissioning replica to
+// replace from the list of voters or non-voters, depending on the replica
+// status and target type, and returns the filtered live voters and non-voters
+// along with the list of existing replicas and the index of the removal candidate.
+// In case of an add action, no replicas are removed and a removeIdx of -1 is
+// returned, and if no candidates for replacement can be found during a replace
+// action, the returned nothingToDo flag will be set to true.
+func DetermineReplicaToReplaceAndFilter(
+	storePool storepool.AllocatorStorePool,
+	action AllocatorAction,
+	voters, nonVoters []roachpb.ReplicaDescriptor,
+	liveVoterReplicas, deadVoterReplicas []roachpb.ReplicaDescriptor,
+	liveNonVoterReplicas, deadNonVoterReplicas []roachpb.ReplicaDescriptor,
+) (
+	existing, remainingLiveVoters, remainingLiveNonVoters []roachpb.ReplicaDescriptor,
+	removeIdx int,
+	nothingToDo bool,
+	err error,
+) {
+	removeIdx = -1
+	remainingLiveVoters = liveVoterReplicas
+	remainingLiveNonVoters = liveNonVoterReplicas
+	var deadReplicas, removalCandidates []roachpb.ReplicaDescriptor
+
+	if !(action.Add() || action.Replace()) {
+		err = errors.AssertionFailedf(
+			"unexpected attempt to filter replicas on non-add/non-replacement action %s",
+			action,
+		)
+		return
+	}
+
+	replicaType := action.TargetReplicaType()
+	replicaStatus := action.ReplicaStatus()
+
+	switch replicaType {
+	case VoterTarget:
+		existing = voters
+		deadReplicas = deadVoterReplicas
+	case NonVoterTarget:
+		existing = nonVoters
+		deadReplicas = deadNonVoterReplicas
+	default:
+		panic(fmt.Sprintf("unknown targetReplicaType: %s", replicaType))
+	}
+	switch replicaStatus {
+	case Alive:
+		// NB: Live replicas are not candidates for replacement.
+		return
+	case Dead:
+		removalCandidates = deadReplicas
+	case Decommissioning:
+		removalCandidates = storePool.DecommissioningReplicas(existing)
+	default:
+		panic(fmt.Sprintf("unknown replicaStatus: %s", replicaStatus))
+	}
+	if len(removalCandidates) == 0 {
+		nothingToDo = true
+		return
+	}
+
+	removeIdx = getRemoveIdx(existing, removalCandidates[0])
+	if removeIdx < 0 {
+		err = errors.AssertionFailedf(
+			"%s %s %v unexpectedly not found in %v",
+			replicaStatus, replicaType, removalCandidates[0], existing,
+		)
+		return
+	}
+
+	// TODO(sarkesian): Add comment on why this filtering only happens for voters.
+	if replicaType == VoterTarget {
+		if len(existing) == 1 {
+			// If only one replica remains, that replica is the leaseholder and
+			// we won't be able to swap it out. Ignore the removal and simply add
+			// a replica.
+			removeIdx = -1
+		}
+
+		if removeIdx >= 0 {
+			replToRemove := existing[removeIdx]
+			for i, r := range liveVoterReplicas {
+				if r.ReplicaID == replToRemove.ReplicaID {
+					remainingLiveVoters = append(liveVoterReplicas[:i:i], liveVoterReplicas[i+1:]...)
+					break
+				}
+			}
+		}
+	}
+	return
+}
+
+func getRemoveIdx(
+	repls []roachpb.ReplicaDescriptor, deadOrDecommissioningRepl roachpb.ReplicaDescriptor,
+) (removeIdx int) {
+	removeIdx = -1
+	for i, rDesc := range repls {
+		if rDesc.StoreID == deadOrDecommissioningRepl.StoreID {
+			removeIdx = i
+			break
+		}
+	}
+	return removeIdx
+}
+
 // ComputeAction determines the exact operation needed to repair the
 // supplied range, as governed by the supplied zone configuration. It
 // returns the required action that should be taken and a priority.
@@ -925,7 +1145,11 @@ func (s *GoodCandidateSelector) selectOne(cl candidateList) *candidate {
 	return cl.selectGood(s.randGen)
 }
 
-func (a *Allocator) allocateTarget(
+// AllocateTarget returns a suitable store for a new allocation of a voting or
+// non-voting replica with the required attributes. Nodes already accommodating
+// voting replicas are ruled out in the voter case, and nodes accommodating
+// _any_ replicas are ruled out in the non-voter case.
+func (a *Allocator) AllocateTarget(
 	ctx context.Context,
 	storePool storepool.AllocatorStorePool,
 	conf roachpb.SpanConfig,
@@ -986,6 +1210,50 @@ func (a *Allocator) allocateTarget(
 	}
 }
 
+// CheckAvoidsFragileQuorum ensures that if we are allocating a new voter and
+// will result in an even number of voters, that we can allocate another voter
+// target in order to avoid a fragile quorum state. This check should be
+// performed whenever we are planning or testing allocation of a new voter.
+//
+// We can skip this check if we're swapping a replica or allocating a non-voter,
+// since that does not change the quorum size.
+func (a *Allocator) CheckAvoidsFragileQuorum(
+	ctx context.Context,
+	storePool storepool.AllocatorStorePool,
+	conf roachpb.SpanConfig,
+	existingVoters, remainingLiveNonVoters []roachpb.ReplicaDescriptor,
+	replicaStatus ReplicaStatus,
+	replicaType TargetReplicaType,
+	newTarget roachpb.ReplicationTarget,
+	isReplacement bool,
+) error {
+	// Validation is only applicable when allocating new voters.
+	if replicaType != VoterTarget {
+		return nil
+	}
+	newVoters := 0
+	if !isReplacement {
+		newVoters = 1
+	}
+	clusterNodes := storePool.ClusterNodeCount()
+	neededVoters := GetNeededVoters(conf.GetNumVoters(), clusterNodes)
+
+	if WillHaveFragileQuorum(len(existingVoters), newVoters, neededVoters, clusterNodes) {
+		// This means we are going to up-replicate to an even replica state.
+		// Check if it is possible to go to an odd replica state beyond it.
+		oldPlusNewReplicas := existingVoters
+		oldPlusNewReplicas = append(
+			oldPlusNewReplicas,
+			roachpb.ReplicaDescriptor{NodeID: newTarget.NodeID, StoreID: newTarget.StoreID},
+		)
+
+		_, _, err := a.AllocateVoter(ctx, storePool, conf, oldPlusNewReplicas, remainingLiveNonVoters, replicaStatus)
+		return err
+	}
+
+	return nil
+}
+
 // AllocateVoter returns a suitable store for a new allocation of a voting
 // replica with the required attributes. Nodes already accommodating existing
 // voting replicas are ruled out as targets.
@@ -996,7 +1264,7 @@ func (a *Allocator) AllocateVoter(
 	existingVoters, existingNonVoters []roachpb.ReplicaDescriptor,
 	replicaStatus ReplicaStatus,
 ) (roachpb.ReplicationTarget, string, error) {
-	return a.allocateTarget(ctx, storePool, conf, existingVoters, existingNonVoters, replicaStatus, VoterTarget)
+	return a.AllocateTarget(ctx, storePool, conf, existingVoters, existingNonVoters, replicaStatus, VoterTarget)
 }
 
 // AllocateNonVoter returns a suitable store for a new allocation of a
@@ -1009,7 +1277,7 @@ func (a *Allocator) AllocateNonVoter(
 	existingVoters, existingNonVoters []roachpb.ReplicaDescriptor,
 	replicaStatus ReplicaStatus,
 ) (roachpb.ReplicationTarget, string, error) {
-	return a.allocateTarget(ctx, storePool, conf, existingVoters, existingNonVoters, replicaStatus, NonVoterTarget)
+	return a.AllocateTarget(ctx, storePool, conf, existingVoters, existingNonVoters, replicaStatus, NonVoterTarget)
 }
 
 // AllocateTargetFromList returns a suitable store for a new allocation of a
diff --git a/pkg/kv/kvserver/replicate_queue.go b/pkg/kv/kvserver/replicate_queue.go
index 588cf6e5cb23..1233bc6444ff 100644
--- a/pkg/kv/kvserver/replicate_queue.go
+++ b/pkg/kv/kvserver/replicate_queue.go
@@ -1003,14 +1003,9 @@ func (rq *replicateQueue) PlanOneChange(
 	// range descriptor.
 	desc, conf := repl.DescAndSpanConfig()
 
-	voterReplicas := desc.Replicas().VoterDescriptors()
-	nonVoterReplicas := desc.Replicas().NonVoterDescriptors()
-	liveVoterReplicas, deadVoterReplicas := rq.storePool.LiveAndDeadReplicas(
-		voterReplicas, true, /* includeSuspectAndDrainingStores */
-	)
-	liveNonVoterReplicas, deadNonVoterReplicas := rq.storePool.LiveAndDeadReplicas(
-		nonVoterReplicas, true, /* includeSuspectAndDrainingStores */
-	)
+	voterReplicas, nonVoterReplicas,
+		liveVoterReplicas, deadVoterReplicas,
+		liveNonVoterReplicas, deadNonVoterReplicas := allocatorimpl.LiveAndDeadVoterAndNonVoterReplicas(rq.storePool, desc)
 
 	// NB: the replication layer ensures that the below operations don't cause
 	// unavailability; see:
@@ -1021,21 +1016,47 @@ func (rq *replicateQueue) PlanOneChange(
 
 	var err error
 	var op AllocationOp
+	removeIdx := -1
+	nothingToDo := false
 	switch action {
 	case allocatorimpl.AllocatorNoop, allocatorimpl.AllocatorRangeUnavailable:
 		// We're either missing liveness information or the range is known to have
 		// lost quorum. Either way, it's not a good idea to make changes right now.
 		// Let the scanner requeue it again later.
 
-	// Add replicas.
-	case allocatorimpl.AllocatorAddVoter:
-		op, err = rq.addOrReplaceVoters(
-			ctx, repl, liveVoterReplicas, liveNonVoterReplicas, -1 /* removeIdx */, allocatorimpl.Alive, allocatorPrio,
-		)
-	case allocatorimpl.AllocatorAddNonVoter:
-		op, err = rq.addOrReplaceNonVoters(
-			ctx, repl, liveVoterReplicas, liveNonVoterReplicas, -1 /* removeIdx */, allocatorimpl.Alive, allocatorPrio,
-		)
+	// Add replicas, replace dead replicas, or replace decommissioning replicas.
+	case allocatorimpl.AllocatorAddVoter, allocatorimpl.AllocatorAddNonVoter,
+		allocatorimpl.AllocatorReplaceDeadVoter, allocatorimpl.AllocatorReplaceDeadNonVoter,
+		allocatorimpl.AllocatorReplaceDecommissioningVoter, allocatorimpl.AllocatorReplaceDecommissioningNonVoter:
+		var existing, remainingLiveVoters, remainingLiveNonVoters []roachpb.ReplicaDescriptor
+
+		existing, remainingLiveVoters, remainingLiveNonVoters, removeIdx, nothingToDo, err =
+			allocatorimpl.DetermineReplicaToReplaceAndFilter(
+				rq.storePool,
+				action,
+				voterReplicas, nonVoterReplicas,
+				liveVoterReplicas, deadVoterReplicas,
+				liveNonVoterReplicas, deadNonVoterReplicas,
+			)
+		if nothingToDo || err != nil {
+			// Nothing to do.
+			break
+		}
+
+		switch action.TargetReplicaType() {
+		case allocatorimpl.VoterTarget:
+			op, err = rq.addOrReplaceVoters(
+				ctx, repl, existing, remainingLiveVoters, remainingLiveNonVoters,
+				removeIdx, action.ReplicaStatus(), allocatorPrio,
+			)
+		case allocatorimpl.NonVoterTarget:
+			op, err = rq.addOrReplaceNonVoters(
+				ctx, repl, existing, remainingLiveVoters, remainingLiveNonVoters,
+				removeIdx, action.ReplicaStatus(), allocatorPrio,
+			)
+		default:
+			panic(fmt.Sprintf("unsupported targetReplicaType: %v", action.TargetReplicaType()))
+		}
 
 	// Remove replicas.
 	case allocatorimpl.AllocatorRemoveVoter:
@@ -1043,67 +1064,6 @@ func (rq *replicateQueue) PlanOneChange(
 	case allocatorimpl.AllocatorRemoveNonVoter:
 		op, err = rq.removeNonVoter(ctx, repl, voterReplicas, nonVoterReplicas)
 
-	// Replace dead replicas.
-	case allocatorimpl.AllocatorReplaceDeadVoter:
-		if len(deadVoterReplicas) == 0 {
-			// Nothing to do.
-			break
-		}
-		removeIdx := getRemoveIdx(voterReplicas, deadVoterReplicas[0])
-		if removeIdx < 0 {
-			err = errors.AssertionFailedf(
-				"dead voter %v unexpectedly not found in %v",
-				deadVoterReplicas[0], voterReplicas)
-			break
-		}
-		op, err = rq.addOrReplaceVoters(
-			ctx, repl, liveVoterReplicas, liveNonVoterReplicas, removeIdx, allocatorimpl.Dead, allocatorPrio)
-	case allocatorimpl.AllocatorReplaceDeadNonVoter:
-		if len(deadNonVoterReplicas) == 0 {
-			// Nothing to do.
-			break
-		}
-		removeIdx := getRemoveIdx(nonVoterReplicas, deadNonVoterReplicas[0])
-		if removeIdx < 0 {
-			err = errors.AssertionFailedf(
-				"dead non-voter %v unexpectedly not found in %v",
-				deadNonVoterReplicas[0], nonVoterReplicas)
-			break
-		}
-		op, err = rq.addOrReplaceNonVoters(
-			ctx, repl, liveVoterReplicas, liveNonVoterReplicas, removeIdx, allocatorimpl.Dead, allocatorPrio)
-	// Replace decommissioning replicas.
-	case allocatorimpl.AllocatorReplaceDecommissioningVoter:
-		decommissioningVoterReplicas := rq.storePool.DecommissioningReplicas(voterReplicas)
-		if len(decommissioningVoterReplicas) == 0 {
-			// Nothing to do.
-			break
-		}
-		removeIdx := getRemoveIdx(voterReplicas, decommissioningVoterReplicas[0])
-		if removeIdx < 0 {
-			err = errors.AssertionFailedf(
-				"decommissioning voter %v unexpectedly not found in %v",
-				decommissioningVoterReplicas[0], voterReplicas)
-			break
-		}
-		op, err = rq.addOrReplaceVoters(
-			ctx, repl, liveVoterReplicas, liveNonVoterReplicas, removeIdx, allocatorimpl.Decommissioning, allocatorPrio)
-	case allocatorimpl.AllocatorReplaceDecommissioningNonVoter:
-		decommissioningNonVoterReplicas := rq.storePool.DecommissioningReplicas(nonVoterReplicas)
-		if len(decommissioningNonVoterReplicas) == 0 {
-			// Nothing to do.
-			break
-		}
-		removeIdx := getRemoveIdx(nonVoterReplicas, decommissioningNonVoterReplicas[0])
-		if removeIdx < 0 {
-			err = errors.AssertionFailedf(
-				"decommissioning non-voter %v unexpectedly not found in %v",
-				decommissioningNonVoterReplicas[0], nonVoterReplicas)
-			break
-		}
-		op, err = rq.addOrReplaceNonVoters(
-			ctx, repl, liveVoterReplicas, liveNonVoterReplicas, removeIdx, allocatorimpl.Decommissioning, allocatorPrio)
-
 	// Remove decommissioning replicas.
 	//
 	// NB: these two paths will only be hit when the range is over-replicated and
@@ -1161,18 +1121,6 @@ func (rq *replicateQueue) PlanOneChange(
 	return change, err
 }
 
-func getRemoveIdx(
-	repls []roachpb.ReplicaDescriptor, deadOrDecommissioningRepl roachpb.ReplicaDescriptor,
-) (removeIdx int) {
-	for i, rDesc := range repls {
-		if rDesc.StoreID == deadOrDecommissioningRepl.StoreID {
-			removeIdx = i
-			break
-		}
-	}
-	return removeIdx
-}
-
 func maybeAnnotateDecommissionErr(err error, action allocatorimpl.AllocatorAction) error {
 	if err != nil && isDecommissionAction(action) {
 		err = decommissionPurgatoryError{err}
@@ -1198,32 +1146,15 @@ func isDecommissionAction(action allocatorimpl.AllocatorAction) bool {
 func (rq *replicateQueue) addOrReplaceVoters(
 	ctx context.Context,
 	repl *Replica,
-	liveVoterReplicas, liveNonVoterReplicas []roachpb.ReplicaDescriptor,
+	existingVoters []roachpb.ReplicaDescriptor,
+	remainingLiveVoters, remainingLiveNonVoters []roachpb.ReplicaDescriptor,
 	removeIdx int,
 	replicaStatus allocatorimpl.ReplicaStatus,
 	allocatorPriority float64,
 ) (op AllocationOp, _ error) {
 	effects := effectBuilder{}
 	desc, conf := repl.DescAndSpanConfig()
-	existingVoters := desc.Replicas().VoterDescriptors()
-	if len(existingVoters) == 1 {
-		// If only one replica remains, that replica is the leaseholder and
-		// we won't be able to swap it out. Ignore the removal and simply add
-		// a replica.
-		removeIdx = -1
-	}
-
-	remainingLiveVoters := liveVoterReplicas
-	remainingLiveNonVoters := liveNonVoterReplicas
-	if removeIdx >= 0 {
-		replToRemove := existingVoters[removeIdx]
-		for i, r := range liveVoterReplicas {
-			if r.ReplicaID == replToRemove.ReplicaID {
-				remainingLiveVoters = append(liveVoterReplicas[:i:i], liveVoterReplicas[i+1:]...)
-				break
-			}
-		}
-	}
+	isReplace := removeIdx >= 0
 
 	// The allocator should not try to re-add this replica since there is a reason
 	// we're removing it (i.e. dead or decommissioning). If we left the replica in
@@ -1233,41 +1164,23 @@ func (rq *replicateQueue) addOrReplaceVoters(
 	if err != nil {
 		return nil, err
 	}
-	if removeIdx >= 0 && newVoter.StoreID == existingVoters[removeIdx].StoreID {
+	if isReplace && newVoter.StoreID == existingVoters[removeIdx].StoreID {
 		return nil, errors.AssertionFailedf("allocator suggested to replace replica on s%d with itself", newVoter.StoreID)
 	}
 
-	clusterNodes := rq.storePool.ClusterNodeCount()
-	neededVoters := allocatorimpl.GetNeededVoters(conf.GetNumVoters(), clusterNodes)
-
-	// Only up-replicate if there are suitable allocation targets such that,
-	// either the replication goal is met, or it is possible to get to the next
+	// We only want to up-replicate if there are suitable allocation targets such
+	// that, either the replication goal is met, or it is possible to get to the next
 	// odd number of replicas. A consensus group of size 2n has worse failure
 	// tolerance properties than a group of size 2n - 1 because it has a larger
 	// quorum. For example, up-replicating from 1 to 2 replicas only makes sense
 	// if it is possible to be able to go to 3 replicas.
-	//
-	// NB: If willHave > neededVoters, then always allow up-replicating as that
-	// will be the case when up-replicating a range with a decommissioning
-	// replica.
-	//
-	// We skip this check if we're swapping a replica, since that does not
-	// change the quorum size.
-	if willHave := len(existingVoters) + 1; removeIdx < 0 && willHave < neededVoters && willHave%2 == 0 {
-		// This means we are going to up-replicate to an even replica state.
-		// Check if it is possible to go to an odd replica state beyond it.
-		oldPlusNewReplicas := append([]roachpb.ReplicaDescriptor(nil), existingVoters...)
-		oldPlusNewReplicas = append(
-			oldPlusNewReplicas,
-			roachpb.ReplicaDescriptor{NodeID: newVoter.NodeID, StoreID: newVoter.StoreID},
-		)
-		_, _, err := rq.allocator.AllocateVoter(ctx, rq.storePool, conf, oldPlusNewReplicas, remainingLiveNonVoters, replicaStatus)
-		if err != nil {
-			// It does not seem possible to go to the next odd replica state. Note
-			// that AllocateVoter returns an allocatorError (a PurgatoryError)
-			// when purgatory is requested.
-			return nil, errors.Wrap(err, "avoid up-replicating to fragile quorum")
-		}
+	if err := rq.allocator.CheckAvoidsFragileQuorum(ctx, rq.storePool, conf,
+		existingVoters, remainingLiveNonVoters,
+		replicaStatus, allocatorimpl.VoterTarget, newVoter, isReplace); err != nil {
+		// It does not seem possible to go to the next odd replica state. Note
+		// that AllocateVoter returns an allocatorError (a PurgatoryError)
+		// when purgatory is requested.
+		return nil, errors.Wrap(err, "avoid up-replicating to fragile quorum")
 	}
 
 	// Figure out whether we should be promoting an existing non-voting replica to
@@ -1291,7 +1204,7 @@ func (rq *replicateQueue) addOrReplaceVoters(
 		})
 		ops = roachpb.MakeReplicationChanges(roachpb.ADD_VOTER, newVoter)
 	}
-	if removeIdx < 0 {
+	if !isReplace {
 		log.KvDistribution.Infof(ctx, "adding voter %+v: %s",
 			newVoter, rangeRaftProgress(repl.RaftStatus(), existingVoters))
 	} else {
@@ -1333,14 +1246,14 @@ func (rq *replicateQueue) addOrReplaceVoters(
 func (rq *replicateQueue) addOrReplaceNonVoters(
 	ctx context.Context,
 	repl *Replica,
+	existingNonVoters []roachpb.ReplicaDescriptor,
 	liveVoterReplicas, liveNonVoterReplicas []roachpb.ReplicaDescriptor,
 	removeIdx int,
 	replicaStatus allocatorimpl.ReplicaStatus,
 	allocatorPrio float64,
 ) (op AllocationOp, _ error) {
-	desc, conf := repl.DescAndSpanConfig()
-	existingNonVoters := desc.Replicas().NonVoterDescriptors()
 	effects := effectBuilder{}
+	conf := repl.SpanConfig()
 
 	newNonVoter, details, err := rq.allocator.AllocateNonVoter(ctx, rq.storePool, conf, liveVoterReplicas, liveNonVoterReplicas, replicaStatus)
 	if err != nil {