The right answer is Southern Democrats split from their party.
In July 1948, during a Democratic Convention in Philadelphia Democrats had two major issues to discuss: A battle over the civil rights program and Truman's acceptance speech. In the matter of civil rights, the program opposed racial discrimination only in general terms, because of southern hostility. The Mayor of the city of Minneapolis signaled the fate of that discussion, positioning on the side of human rights. White segregationists delegates from 2 southern states, Alabama and Mississippi split from their party for the first time since the end of the Civil War.
