What issue split the southern states from being solidly democrat?

The issue of the exclusion of blacks from the voting system in the primaries. after the 1960's, however, the Voting Rights Act of 1965 entrenched the rights of African American voters in the political system, joining the political system for the first time in the 20th Century. As black people who had remained in the region became affiliated with the Democratic Party, so more White Americans moved towards supporting the Republican Party, creating a stark split.