A model of political competition with citizen-candidates

We develop a model of electoral competition in which citizens choose whether or not to run as candidates. A winner implements her favorite policy. The equilibrium number of candidates depends negatively on the cost of running and positively on the benefits of winning. For some parameter values all equilibria under plurality rule have exactly two candidates, whose positions are distinct. Two-candidate elections are more likely under plurality rule than under a runoff system (cf. Duverger's Law). The candidates' positions are less differentiated under a runoff system. There exist equilibria under both systems in which some candidates have no chance of winning.