What explains party unity in legislatures? Prior research suggests when candidate selection and electoral rules create incentives for legislators to cultivate a personal vote, party unity should decline. However, previous theories often treat candidate selection rules as exogenous institutions that have independent effects on legislative behavior despite the fact the choice of nomination rules are often an intraparty issue and a key component of partisan electoral strategies. Here, we develop a theory linking candidate selection rules to party unity through the strategic behavior of party leaders. Our main finding is that, under personalized electoral rules, the effects of nomination rules on legislative behavior are conditional on parties' electoral incentives. We test our theory using data from the Mexican Chamber of Deputies, a case where our results are unlikely to be explained by personal vote‐seeking incentives since legislators are banned from seeking reelection.

How do parties protect themselves from electoral manipulation? To answer this question, we study the drivers of polling station party representatives’ presence and their impact on electoral outcomes in an environment where electoral irregularities are common. Using election data from the Mexican Chamber of Deputies, we find a robust positive correlation between the presence of party representatives and that party’s vote share. The evidence suggests that this correlation can be attributed to party representatives influencing the electoral results. We also formulate a game theoretic model of the levels of representation chosen by parties in a given precinct and structurally estimate its parameters. We find that parties send their representatives where they expect their opponents to send their own. The finding suggests representatives play a primarily protective role, even when they are often involved in irregularities themselves.

Why do party leaders constrain their own power and allow the use of primary elections? I develop a model of intraparty politics and electoral competition in which an ambitious office-seeker reevaluates their party affiliation after their party chooses a nomination rule. The model shows that in settings in which the linkages between politicians and parties are weak, party leaders adopt primaries in order to keep potential defectors from pursuing their individual ambitions outside of the party. The main theoretical result shows that only parties that are electorally strong ex ante can use primaries as a strategy to hold their parties together. Thus, in contrast to an extensive literature linking the use of primaries to electorally weak parties, this model posits the existence of a positive relationship between the use of primaries and a party's electoral strength. Analysis of legislative and gubernatorial nominations in Mexico provides strong support for the model's expectations.Verification Materials: The data and materials required to verify the computational reproducibility of the results, procedures, and analyses in this article are available on the American Journal of Political Science Dataverse within the Harvard Dataverse Network, at: https://doi.org/10.7910/DVN/GRVR99. I n July 2010, Mario López Valdez, popularly known as "Malova," won a groundbreaking gubernatorial election that ended over eight decades of uninterrupted rule by the Partido Revolucionario Institucional (PRI) in the Mexican state of Sinaloa. This outcome came as a surprise for many observers of Mexican politics. Just six months prior, the real struggle for power was taking place within the PRI itself, with Malova, then a member of that party, trying to win the party's nomination over Jesús Vizcarra, the sitting governor's choice to succeed him in office. By late March, when it became clear that Malova would not become the PRI's candidate, he left the party and secured the nomination of a pre-electoral coalition formed by the conservative Partido Acción Nacional (PAN) and the leftist Partido de la Revolución Democrática (PRD). Although the circumstances surrounding this election might seem unusual, cases like this are common in Mexican politics (Barrow 2007;Kerevel 2014). 1 This Mexican election illustrates a central feature of electoral and legislative politics in many developing countries and emerging democracies-the weakness of politicians' attachments towards political parties (Desposato 2006;Hicken 2006;Shabad and Slomczynski 2004). The fragility of party loyalties poses formidable challenges for political parties. 2 Even in settings where

Over recent decades, an increasing number of presidential candidates across the world have been nominated in primary elections. Yet, our understanding of presidential primaries outside of the U.S. remains very limited. This paper advances a theoretical argument linking presidential primary outcomes to the strategic behavior of subnational party elites, particularly those with the resources to mobilize primary voters in their regions. I provide support for this claim using an original dataset from the 2012 presidential primary of the Mexican Partido Acción Nacional (PAN), in which one of the candidates was very closely linked to (and informally endorsed by) the PAN leadership. The results show the levels of support for this candidate were higher in places with PAN officeholders. Additional evidence suggests this pattern is driven by PAN officeholders (1) investing resources into electoral mobilization and (2) tampering with the lists of registered voters.