Can subnational elections contribute to democratization? In autocracies that hold competitive elections at multiple levels of government, subnational executive offices provide opposition parties with access to resources, increase their visibility among voters, and let them gain experience in government. This allows opposition parties to use subnational executives as "springboards" from which to increase their electoral support in future races, and predicts that their electoral support should follow a diffusion process, that is, a party’s electoral performance in municipality m at time t should be better if that party already governs some of m’s neighbors since t – 1. I evaluate this claim with data from municipal-level elections in Mexico between 1984 and 2000. Consistent with the fact that the Partido Acción Nacional (National Action Party, PAN) followed an explicit strategy of party-building from below but the Partido de la Revolución Democrática (Party of the Democratic Revolution, PRD) did not, the results indicate that diffusion effects contributed to the growth of the former but not the latter.