The affective, identity based, and often negative nature of partisan polarization in the United States has been a subject of much scholarly attention. Applying insights from recent work in social psychology, we employ three novel large-N, broadly representative online surveys, fielded over the course of 4 years, across two presidential administrations, to examine the extent to which this brand of polarization features a willingness to apply dehumanizing metaphors to out-partisans. We begin by looking at two different measures of dehumanization (one subtle and one more direct). This uncovers striking, consistent observational evidence that many partisans dehumanize members of the opposing party. We examine the relationship between dehumanization and other key partisan intensity measures, finding that it is most closely related to extreme affective polarization. We also show that dehumanization “predicts” partisan motivated reasoning and is correlated with respondent worldview. Finally, we present a survey experiment offering causal leverage to examine openness to dehumanization in the processing of new information about misdeeds by in- and out-partisans. Participants were exposed to identical information about a melee at a gathering, with the partisanship of those involved randomly assigned. We find pronounced willingness by both Democrats and Republicans to dehumanize members of the out-party. These findings shed considerable light on the nature and depth of modern partisan polarization.