This study analyzes the effect of viewing a presidential campaign debate on political polarization. The results of quasi-experimental debate studies of all presidential general election debates in 2000, 2004, 2008, and 2012 as well as vice presidential debates in 2008 and 2012 demonstrate a consistent effect: Viewing a debate increased political polarization. However, predebate levels of polarization moderated this effect such that those viewers with very little polarization experienced the most significant increase and those who were highly polarized prior to viewing a debate experienced no significant change. Overall, our findings contribute to a growing body of research on the polarizing effects of campaign communication and raise important questions about how these effects should be interpreted in future research.