We examine the electoral alliances between two Mexican political parties – the Partido Revolucionario Institucional (PRI) and the Partido Verde Ecologista de México (PVEM). Despite the PRI’s electoral dominance, it has entered into preelection agreements with the PVEM since 2003. These electoral pacts are unusual for several reasons: the parties do not share an ideology; their bases of support come from different social sectors; and the PRI’s survival as a party does not depend on these pacts. Using electoral data from 2006 to 2015, we examine the electoral districts in which the PRI and the PVEM ran joint candidates in federal legislative elections. We find that the ultimate goals of each party, their past electoral performances at the legislative district level, and the presence of PRI–PVEM alliances in gubernatorial elections explain the parties’ choices to collaborate in certain districts. Our findings have important implications for understanding the behavior of parties in newer democracies.