Many Americans’ political identity is more relevant, salient, and central than their American identity. Political parties provide party members with an identity and belief system. Political identities influence identity-based ideologies which increase affective polarization and potential anti-social intergroup conflict between political parties. The purpose of the current research is to determine how political party identification and ideological extremity influences affective polarization for Republicans and Democrats across two studies. Participants in both studies completed a survey which measured political identification, ideological extremity, and uniquely operationalized measures of affective polarization, including support for co-partisans, attitudes about political rivals, emotions towards political rivals, action tendencies towards political rivals, and perceived threat from political rivals. Study 1 (N = 530) used responses to the survey from a convenience sample crowdsourced by Amazon Mechanical Turk (AMT) in 2018. Study 1 showed that higher political party identification is associated with significantly higher affective polarization for members of both parties and that ideological extremity partially mediates this effect. Study 2 (N = 260) used a 2020 AMT sample to replicate Study 1’s results. Findings extend our understanding of identity-based ideological extremity and suggest that affective polarization could be driven by political party identification and partisans’ ideological extremity.