Candidates attempt to mobilize supporters and persuade swing voters to win elections. I derive theoretical propositions that suggest that the influence of swing voters depends upon change in the candidates across elections. The consequences of changes in composition depend upon the relative balance of campaign expenditures. I estimate a Bayesian hierarchical model on Florida electoral data for house, governor, and senate contests. Swing voters contribute on average 4.1 percentage points to change in party vote shares, while change in turnout influences outcomes by 8.6 points. The effect of swing voters is increasing in the divergence between the Democrat and Republican candidates, and candidates increasingly benefit from the votes of occasional voters as the relative balance of campaign spending increases in their favor. More broadly, I show that the effects of swing voters and turnout are not constant features of American elections. Instead, these effects vary across time and space in ways that can be explained by candidates and context.