This paper explores the impacts of electoral institutions on citizens’ social networks and political engagement, using the case of Brazil. Brazil’s combination of open-list proportional representation and extreme multipartism leads to high numbers of connections between citizens and local politicians and activists. Survey data from a 2008 city council race reveal that a very high percentage of respondents know both politicians and activists. Such connections serve as an important source of political socialization and mobilization. Using coarsened exact matching, I show that these ties affect campaign learning, turnout, and clientelistic dispositions, and that they often have a more powerful effect than do respondents’ closest discussants. This paper thus illuminates a hitherto unrecognized consequence of Brazil’s much-studied and distinctive institutional arrangements, while at the same time developing a new framework for theorizing and measuring the ways in which citizens’ networks incorporate politicians.