This article studies a model of political parties as informative "brands" to voters. Voters across a large number of constituencies are assumed to be risk averse and incompletely informed about candidate ideal policies, and candidates are unable to commit to a declared policy platform. In this environment, parties can play a critical role by aggregating ideologically similar candidates and signaling their preferences to voters. This signaling is effective because party membership imposes costs, which screen out candidates whose preferences are not sufficiently close to the party's platform. We find that when party labels are very informative, the parties' platforms converge. When party labels are less informative, however, platforms diverge, because taking an extreme position allows a party to reduce the variance of its members' preferences. As parties become less able to impose costs on their members, or less able to screen out certain types of candidates, their platforms move further apart.
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