Endogenous Party Structure

This paper proposes a model of two-party representative democracy on a single-dimensional political space, in which voters choose their parties in order to influence the parties' choices of representative. After two candidates are selected as the median of each party's support group, Nature determines the candidates' relative lkeability (valence). Based on the candidates' political positions and relative likeability, voters vote for the preferable candidate without being tied to their party's choice. We show that (1) there exists a nontrivial equilibrium under natural conditions, and that (2) the equilibrium party border and the ex ante probabilities of the two-party candidates winning are sensitive to the distribution of voters. In particular, we show that if a party has a more concentrated subgroup, then the party tends to alienate its centrally located voters, and the party's probability of winning the final election is reduced. Even if voter distribution is symmetric, an extremist party (from either side) can emerge as voters become more politically divided.