Today, Black Americans are the strongest Democratic constituency and White Southerners are the strongest Republican group—but it used to be the other way around. The usual story places 1960s civil rights policymaking at the center of the switch, but an important prior history in the North and the South made it possible. Keneshia Grant finds that the Great Migration north changed the Democratic Party because Black voters became pivotal in Democratic cities like New York, Chicago, and Detroit, leading politicians to respond, including new Black elected officials. Boris Heersink finds that Southern Republican state parties became battles between racially mixed and lily-white factions, mostly for control of patronage due to national convention influence. The lily-white takeovers enabled early Republican gains in the South.These trends predated national civil rights policymaking and help explain how we reached today’s divided regional and racial politics.