Single-round presidential elections are thought to depress the effective number of candidates, while the presence of runoff rules is expected to draw more candidates to the race. Empirical evidence in support of this proposition has been gathered on small to medium-N samples of presidential democracies, although contextual variables vary widely across cases. We leverage the existence of 5,570 subnational presidential systems (municipalities) in Brazil, in which directly elected presidents (mayors) have competed since 1992 under a unique rule: runoff elections are held only in municipalities with more than 200,000 voters. We use a regression discontinuity design to analyse candidate supply in cities just above and below this threshold. We leverage another institutional change, permitting consecutive reelection of mayors since 2000. Controlling for incumbency, party system variables, ideology, campaign finance, and candidate quality, we shed further light on the depressive force of runoff rules on the effective number of presidential candidates.
