We estimate the effects of one of the largest anti-vote-buying campaigns ever studied—with half a million voters exposed across 1427 villages—in Uganda’s 2016 elections. Working with civil society organizations, we designed the study to estimate how voters and candidates responded to their campaign in treatment and spillover villages, and how impacts varied with campaign intensity. Despite its heavy footprint, the campaign did not reduce politician offers of gifts in exchange for votes. However, it had sizable effects on people’s votes. Votes swung from well-funded incumbents (who buy most votes) towards their poorly-financed challengers. We argue the swing arose from changes in village social norms plus the tactical response of candidates. While the campaign struggled to instill norms of refusing gifts, it leveled the electoral playing field by convincing some voters to abandon norms of reciprocity—thus accepting gifts from politicians but voting for their preferred candidate.