Even after the ability to enact racist laws had become illegal, the old guard dems in the south remained loyal Democrats all their lives. The younger voters in the South started to vote Republican, because the Democratic Party was seen as steadily lurching to the left, and now look at it.
I gave you the example of Robert Byrd, the man was a racist SOB his entire life, and yet the dems made him their Senate leader from 1977 to 1989. If the dems were magically no longer racists, why did the entire old guard remain, except Strom? Why why did the Senate dems keep electing a flaming racist Byrd as their leader? the racist old guard were losing elections to Republicans, and it was not because these Republicans were more bigoted than the old guard racial bigots.
It's just silly to try and make the argument that the racists suddenly decided that the only political party they could use as their vehicle to enact their racist laws thru, was the very political party that has always fought against their racist laws.