The chapter explores the causes of polarization in the United States and their relationship to support for Trump. The conventional explanation for “Trumpism” is dispositional, i.e. attraction to Candidate Trump is attributable to the values and attitudes of votersVoters. But as the chapter argues, the degree of ill will across the party divide is an additional contextual explanation, making social identities politicized and partisans especially responsive to a candidate who aggressively attacks and demeans key opposing constituencies. The psychological mechanism underlying this effect is the stirring of votersVoters’ partisan instincts through hostile rhetoric and actions targeting groups aligned with the parties. By attacking Latinos, women, and Blacks, groups seen as proDemocrat, Trump has capitalizd on Republicans’ sense of “us against them.” The experiments by Peterson and Westwood provide the first unequivocal evidence that hostility toward members of a racial out group is sufficient to elicit hostility toward the political party aligned with that group. The chapter suggests the future research agenda will need to cast a wider net than before and investigate affective spillover involving multiple identities and group-based attacks by candidates from both parties.