This second chapter on mechanisms argues that competitive elections cause machines to target many unbuyable voters with electoral gifts. Vote buying is effective when recipients vote for the machine but would have voted for a rival party had they not received a gift. Existing research shows that machines select clients based on citizens’ observable pre-campaign dispositions. Yet the unknowable effects of the subsequent campaigns can shift recipients’ vote-relevant attitudes enough to make some initially buyable voters unbuyable by Election Day. Machines in competitive constituencies thus make many unavoidable targeting errors. Campaign effects are muted in constituencies that the machine already dominates electorally, leading to more efficient targeting and more effective vote buying. Tests use a variety of measurement techniques and election-season panel surveys from Mexico between 2000 and 2018.