This article examines when partisan media effects occur during presidential campaigns. I argue that partisan media are most likely to influence candidate impressions early in the election cycle, when voters have less crystallized impressions of the candidates and are less motivated to defend their party’s nominee. Using multiple methods and two large-scale surveys spanning 2008, I show that Fox News affected favorability toward Barack Obama during the first five months of the election year, but those effects largely disappeared over the last five months. The results varied by political knowledge, however, as Fox News affected low-knowledge viewers throughout the entire year, but only affected high-knowledge viewers early in the election cycle. These results provide important new evidence on how partisan media affect viewers and when those effects occur during a presidential election.