--- - |2+ Abstract Objective We investigate how the relations between the Big Five personality traits and political preferences develop over a campaign. Method We rely on a six‐wave nationwide longitudinal survey from the 2008 U.S. election that included 20,000 respondents (Mage = 49, SD = 15; 53% women, 47% men; 82% White, 8% Black, 6% Hispanic/Latino, 1% Asian, 1% Native American, 2% other). Survey weights were applied to approximate a representative sample of the U.S. population. Ns for reported analyses range from 5,160 to 12,535. Results First, Conscientiousness and Openness to Experience were significantly associated with changes in outcomes over time, such that individuals higher in Conscientiousness and lower in Openness tended to become more conservative, identify as more Republican, and evaluate John McCain more favorably relative to Barack Obama. Second, the effects of personality on candidate evaluations were mediated by partisanship and ideology. Finally, we find that the relations between traits and late‐campaign candidate evaluations are stronger than those between traits and early‐campaign candidate evaluations. Conclusions Personality plays an important, dynamic role in the formation and change of political preferences over the course of political campaigns—a role not entirely visible in cross‐sectional analyses. - Journal of Personality, EarlyView.