Loud and vitriolic campaigns are increasingly the norm. For an electorate for which partisanship is a salient social identity, campaign messages questioning the intentions, integrity, and patriotism of political opponents are liable to not only reinforce partisans’ stereotypes of the other side, but also engender new negative stereotypes. We use data from multiple large national surveys, and the Wisconsin Advertising Project to demonstrate that partisans’ evaluations of their opponents become more negative over the course of the campaign. Exposure to televised political advertising, especially negative advertising, increases partisan a ffect. We discuss the implications of our findings for current debates about the extent of partisan polarization within the mass public, and the consequences of such polarization for electoral accountability.