We test if within-person changes in political identities are associated with within-person changes in political animosity in two longitudinal studies (United States N = 552, Waves = 26; Netherlands N = 1,670, Waves = 12). Typical studies examine cross-sectional associations without assessing within-person change. Our work provides a stronger test of the relationship. We find that within-person changes in the strength of people’s ideological and partisan identities are associated with increased political animosity. We found no such associations with within-person changes in identity direction. These patterns were robust to covariates and emerged in both studies. In addition to these average effects, we found substantial heterogeneity across participants in the associations among identity strength, identity direction, and political animosity. We did not find robust and replicable moderators for this heterogeneity. These findings suggest that identity strength (but not identity direction) is a key, if ...