Party Polarization in Legislatures with Office-Motivated Candidates Mattias Polborn∗ James M. Snydery August 11, 2016 Abstract We develop a theory of legislative competition in which voters care about local candidate valence and national party positions that are determined by the parties' median legislators. As long as election outcomes are sufficiently predictable, the only stable equilibria exhibit policy divergence between the parties. If the degree of uncertainty about election outcomes decreases, and if voters place less weight on local candidates' valence, polarization between the parties increases. Furthermore, a systematic electoral shock makes the party favored by the shock more moderate, while the disadvantaged party becomes more extreme. Finally, we examine data on state elections and the ideological positions of state legislatures and find patterns that are consistent with key predictions of our model. Keywords: Legislative competition, polarization. ∗Department of Economics and Department of Political Science, Vanderbilt University. Mailing address: 415 Calhoun Hall, Vanderbilt University, Nashville, TN 37240. E-mail:
[email protected]. yDepartment of Government, Harvard University, and NBER. Mailing address: 1737 Cambridge St., Cambridge, MA, 02138. E-mail:
[email protected] 1 1 Introduction What is the nature of political parties in representative democracies? In the standard spatial model parties are teams, groups of politicians united to win control of government (Downs, 1957). All politicians within a party have the same induced preferences over the party's policy positions or ideology. In practice, however, the politicians in many parties do not exhibit such extreme ideo- logical unity. Dissent frequently arises within parties about the legislative agenda or party platform, in policy proposals, speeches, convention battles, and|at least in the U.S.|roll call votes.1 One possible reason for intra-party disagreement is that politicians care explicitly about policy and simply differ in their personal preferences.