We estimate the effects of a large-scale, randomized grassroots campaign designed to combat vote-buying in the 2016 election in Uganda. Our design and data collection allow us to estimate how candidates and their brokers respond to the campaign in treatment and spillover areas and how the effects of the campaign vary with local treatment intensity. Contrary to our expectations, the campaign did not reduce the extent to which voters accepted cash and gifts in exchange for their vote. However, it led opposition candidates to increase their vote-buying and policy-campaigning efforts, and it had sizeable effects on electoral outcomes, with opposition candidates benefiting from the campaign at the expense of incumbent candidates. Consistent with these effects, we present evidence that the campaign diminished the effectiveness of vote-buying transactions by shifting local social norms against vote-selling and by convincing some voters to vote according to their conscience, regardless of any gifts received.