Partisan affective polarization, measured with feeling thermometer ratings, has increased gradually in the United States over a long period. This article describes how affective polarization and its composite parts, rival-party and own-party feelings, have changed over time. It identifies three analytically distinct processes: sorting, which entails a change in group composition; entrenchment, or an increasing gap between aligned and misaligned copartisans; and fortification, a general change in party feelings across partisan subgroups. While scholars often emphasize the importance of sorting, a Blinder-Oaxaca decomposition analysis of ANES data shows that entrenchment and fortification explain a larger share of these thermometer trends. Furthermore, asymmetries between the two major parties exist: the lion’s share of colder rival-party feelings among Republicans is centered on race, while Democrats’ rival-party feelings grew similarly cold regardless of their race, religion, or ideological extremity. In addition, the gap in party feelings between well and poorly aligned Democrats appears to have decreased over time. Finally, data from two ANES panels suggest that the same partisans’ feelings are growing colder, not that partisans with warm rival-party feelings are switching parties. These findings have important implications for the study of affective polarization and suggest avenues for future research.