We report the results of the first large-scale experiment involving paid political advertising. During the opening months of a 2006 gubernatorial campaign, approximately $2 million of television and radio advertising on behalf of the incumbent candidate was deployed experimentally. In each experimental media market, the launch date and volume of television advertising were randomly assigned. In order to gauge movement in public opinion, a tracking poll conducted brief telephone interviews with approximately 1,000 registered voters each day and a brief follow-up one month after the conclusion of the television campaign. Results indicate that televised ads have strong but short-lived effects on voting preferences. The ephemeral nature of these effects is more consistent with psychological models of priming than with models of on-line processing.