Few transformations have been more significant in American politics in recent decades than the Democratic Party's embrace of racial liberalism and Republicans' adoption of a more conservative stance towards civil rights-related policies. We hypothesize that pressure to embrace a liberal position on civil rights was much stronger among northern Democrats and their coalitional partners than among northern Republicans and their affiliated groups by the mid-1940s, as the Democrats became firmly identified as the party of economic liberalism and labor unions. To test this hypothesis and develop a more fine-grained understanding of the dynamics of party positioning on civil rights, we collect and analyze a new data source: state political party platforms published between 1920 and 1968. These unique data suggest that Democrats had generally become the more liberal party on civil rights by the mid-to-late 1940s across a wide range of states. Our findings – which contradict Carmines and Stimson's prevailing issue evolution model of partisan change – suggest that there were strong coalitional and ideological pressures that led the Democrats to embrace racial liberalism. This finding not only leads to a revised perspective on the civil rights revolution, but also to new insights into the dynamics of partisan realignment more generally.