Dinesh D'Souza had a great explanation, but I've had a hard time finding a video clip. I'm commenting in the hope this jogs another another anon's memory and they can chime in.

Edit: Found a link where he explains it, but it's not short - https://youtu.be/yu2VsZPplug

TLDR: Southern states were racist and Democrat. Conventional wisdom is they stayed racist and became Republican (ergo R's are racist.) BUT, Dinesh points out that racist attitudes were actually declining prior to the Civil Rights Act and that as the South became LESS racist, they became MORE Republican.

Dinesh D'Souza had a great explanation, but I've had a hard time finding a video clip. I'm commenting in the hope this jogs another another anon's memory and they can chime in.