Little is known about the evolution of electoral sentiment over the campaign cycle. How does the outcome come into focus as the election cycle evolves? Do voters’ preferences evolve in a patterned and understandable way? What role does the election campaign play? In this article, we address these issues. We translate general arguments about the role of campaigns into a set of formal, statistical expectations. Then, we outline an empirical analysis and examine poll results for the 15 U.S. presidential elections between 1944 and 2000. Our analysis focuses specifically on two questions. First, to what extent does the observable variation in aggregate poll results represent real movement in electoral preferences (if the election were held the day of the poll) as opposed to mere survey error? Second, to the extent polls register true movement of preferences owing to the shocks of campaign events, do the effects last or do they decay? Answers to these questions tell us whether and the extent to which campaign events have effects on preferences and, if so, whether these effects persist until Election Day. The answers thus inform us about what we ultimately want to know: Do campaigns have any real impact on the election outcome? The results of our analysis suggest that they do.