This dissertation establishes the presence of partisan political intolerance among the American public. This form of political intolerance specifically targets members of the opposing mainline party and the partisans aligned with it. Additionally, it demonstrates that partisan intolerance is a strong predictor of many of the most troublingly anti-democratic attitudes in the American public, including but not limited to support for political hardball, preference for authoritarian rule, and approval of political violence. While observational, these findings are strongly suggestive that partisan political intolerance lies at the root of many of the most troubling trends in American politics.

In addition to the empirical work on the presence and effects of partisan political tolerance, this dissertation also establishes a significant theoretical framework for new measures of political tolerance. These measures align more closely with Sullivan's conception of political intolerance, departing from the predominant yet less favored method used over the past 40 years. Moreover, this dissertation introduces a fresh concept of political tolerance itself: valuing the democratic rights of one’s political opponents above one's own political or policy preferences.