How do politicians buy votes in secret ballot elections? I present a model of vote buying in which a broker sustains bribed voter’s compliance by conditioning future bribes on whether her candidate’s votes reach an optimally set threshold. Unlike previous explanations of compliance, the threshold mechanism does not require brokers to observe individual voter’s political preferences or even vote totals of the bribed voters. I show that when there is uncertainty about voter’s preferences, compliance can be sustained as long as electoral results of small groups are available. If preferences are observed however, vote buying is not deterred by higher aggregation of electoral results. I also find that vote buying is facilitated when voters care about the welfare of other voters. Using survey data from Nigeria, I provide evidence consistent with the model’s results.