Abstract Politics is increasingly a major source of social division, and party identities are theorized to be major drives of political hostility. However, parties often contain factions who are deeply hostile towards one another. Currently, we do not know whether hostility between factions within parties can be as intense as hostility between parties. In this article we compare, for the major parties in Britain ( N s = 522; 568) and the United States ( N = 443), the affect that partisans feel towards factions within their own party and factions in rival parties. We find that within‐party affective polarization effects are large on average ( d = 0.8) and sometimes very large ( d > 1.2), that they are usually smaller than between‐party effects but can equal or exceed them, and, in several cases, factions prefer an out‐party faction over their in‐party rivals. These findings demonstrate that strong affective polarization can emerge between groups who share party identities, highlighting the importance of factions in political psychology and raising questions about the effect of party identities on affective polarization.