Play Nice, Party Hard

Political parties often appear united despite containing factions with divergent preferences. We study why party defections are rare by opening the intra-party 'black box' through a set of game-theoretical models. In the model, the leadership sets the party's policy platform anticipating exit threats from co-partisan rival factions. Rivals weigh policy proximity against the benefits of belonging to a larger party: smaller factions lack credible exit threat and remain loyal, while larger factions may require policy compromises. We emphasise the role of party size and show that unity can be sustained through a snowballing mechanism: once the leadership compromises and secures the loyalty of some factions, other factions become more willing to remain — even with no political concessions — because doing so places them with a larger and more powerful party. Leaders need only win part of the party to induce broader unity, implying that even internally diverse parties can remain stable.