I think you are thinking of the whigg party and the democrats. That was long before the Civil Rights Movement. The big shift during the time was black voters switching to democrats because of republican speeches against the Civil Rights Act.
https://www.studentsofhistory.com/ideologies-flip-Democratic-Republican-parties
The parties switched up during the Civil Rights Movement. Why does the map include both before and after that time?
I think you are thinking of the whigg party and the democrats. That was long before the Civil Rights Movement. The big shift during the time was black voters switching to democrats because of republican speeches against the Civil Rights Act. https://www.studentsofhistory.com/ideologies-flip-Democratic-Republican-parties