This study uses multiwave panel data from the 2008 presidential election to investigate how partisan news exposure influences vote choice over time. Building on Lazarsfeld and colleagues' framework, it distinguishes among three potential effects: activation (motivating partisans to shift back to their own candidate), conversion (motivating shifts to the opposing candidate), and reinforcement (strengthening initial preferences). The findings reveal modest evidence for reinforcement but show partisan news exposure has a stronger influence on vote changes. Exposure aligned with citizens' own partisanship increased odds of activation while decreasing those of conversion, suggesting partisan media plays an active role in shifting political behavior rather than merely confirming it.