To aid the formal verification of fault-tolerant distributed protocols, we propose an approach that significantly reduces the costs of their model checking. These protocols often specify atomic, process-local events that consume a set of messages, change the state of a process, and send zero or more messages. We call such events quorum transitions and leverage them to optimize state exploration in two ways. First, we generate fewer states compared to models where quorum transitions are expressed by single-message transitions. Second, we refine transitions into a set of equivalent, finer-grained transitions that allow partial-order algorithms to achieve better reduction. We implement the MP-Basset model checker, which supports refined quorum transitions. We model check protocols representing core primitives of deployed reliable distributed systems, namely: Paxos consensus, regular storage, and Byzantine-tolerant multicast. We achieve up to 92% memory and 85% time reduction compared to model checking with standard unrefined single-message transitions. © 2011 IEEE.