Democrats spread the myth that the parties switched identities in the 1960s or so. That's a myth, propaganda repeated over and over, aka a Big Lie.
The Dems moved minorities off the plantations or, later, shareholder shacks to the cities. Using the old political machine methods used for early immigrants, the Dems ensnared the minorities with welfare and such to ensure votes. View the large cities managed for decades by Democrats, minorities in ghettoes, fueled by drugs and cheap booze, poor schools, with few options but for the best to escape, the rest to exist in some version of criminality or poor paying jobs. If, there were only a few instances, such could be called happenstance. For it to be the rule involves conscious choices by the Democrat politicians -- parasites, the lot of them.