Primaries: the unifying force

We present a formal model of intra-party politics to explain candidate selection within political parties. We think of parties as heterogeneous groups of individuals who aim to implement a set of policies but who differ in their priorities. When party heterogeneity is too great, parties are in danger of splitting into smaller yet more homogeneous political groups. In this context we argue that primaries can have a unifying role if the party elite cannot commit to policy concessions. Our model shows how three factors interact to create incentives for the adoption of primary elections, namely (1) the alignment in the preferred policies of various factions within a party, (2) the relative weight of each of these factions and (3) the electoral system. We discuss the existing empirical literature and demonstrate how existing studies can be improved in light of our theoretical predictions to provide a new, structured perspective on the adoption of primary elections.