How do voters come to support new political parties? This article contends that new types of locally organized, participant-based societal organizations-such as neighborhood associations, informal sector unions, and indigenous movements-can play a crucial mediating role in securing electoral support for new parties. Drawing on social identity and self-categorization theory, I argue that endorsements of new parties by such organizations sway the vote preferences of organization members and people in their larger social networks. A discrete choice experiment, presenting voters in Bolivia with campaign posters, demonstrates that organizational endorsements are highly effective in mobilizing voters, especially when voters face a new party. Endorsements can even counteract policy and ethnic differences between candidates and voters. The findings suggest an important, understudied route to partisan support in new democracies and have important implications for research on political accountability.