The impact of a campaign on voters' decisions depends, in part, on when voters make their decisions. Voters who decide at the start of a campaign will be much less influenced by the campaign than those who decide at the end. But what explains when voters make their decisions? We address this question by focusing on two key factors: (a) the nature of the campaign information environment - that is, the extent to which it is one-sided or partial to one candidate as opposed to mixed with equally favorable information for both candidates, and (b) individual openness to persuasion - specifically, the extent to which individuals are ambivalent about the candidates. We find that mixed information delays the decisions of voters who are the most ambivalent, above and beyond voters' demographics, political knowledge, interest, and partisan strength. In short, timing of vote decisions depends on an interaction between the competitive nature of the campaign and individual susceptibility to persuasion.