A model of pre-electoral coalition formation

In many countries, three or more candidates compete against one another in single-office elections. I present a model of three candidates competing for a single office in which two candidates can form a coalition prior to the election (a pre-electoral coalition). Since the candidates are both policy- and office-motivated, one candidate can incentivize the other candidate to withdraw his candidacy by choosing a joint policy platform. I examine how electoral environments such as election rules, ideological distance, and pre-election polls influence incentives to form pre-electoral coalitions. I find that pre-electoral coalitions are more likely to form (i) in plurality elections than in two-round runoff elections, (ii) as the threshold for first-round victory decreases in two-round runoff elections, (iii) as the two potential coalition partners' ideological distance increases, conditional on divided support, and (iv) as the fraction of office value which is transferable increases. Moreover, I analyze and compare voter welfare under plurality and the two-round runoff rules.