Primaries: the unifying force

This article presents a formal model of intra-party politics. We think of parties as heterogeneous groups of individuals that aim at implementing a set of policies but differ in their priorities. Our results suggest that this heterogeneity of preferences plays a crucial role for party structure. When party heterogeneity is too large, parties are in danger of splitting into smaller but more homogeneous groups. We show in this context that primaries can have a unifying role. For example, primaries are introduced when a minoritarian party elite fears that a second group will split from the party. As a first step to test this idea, we suggest the use of roll-call data from the US congress to measure party factionalization on the state level. Some preliminary results are presented.