Do presidential campaign advertisements mobilize, inform, or persuade citizens? To answer this question we exploit a natural experiment, the accidental treatment of some individuals living in nonbattleground states during the 2000 presidential election to either high levels or one-sided barrages of campaign advertisements simply because they resided in a media market adjoining a competitive state. We isolate the effects of advertising by matching records of locally broadcast presidential advertising with the opinions of National Annenberg Election Survey respondents living in these uncontested states. This approach remedies the observed correlation between advertising and both other campaign activities and previous election outcomes. In contrast to previous research, we find little evidence that citizens are mobilized by or learn from presidential advertisements, but strong evidence that they are persuaded by them. We also consider the causal mechanisms that facilitate persuasion and investigate whether some individuals are more susceptible to persuasion than others.