
Something to take into consideration was that for the longest time, US politics were more regional which led to the existence of progressives and conservatives within both parties. The South tended to be more conservative while areas like New England and some western/midwestern states like California and Minnesota respectively tended to be more progressive
I think multiple smaller re-alignments over time led to what could be called a full switch. It’s not like one day in 1964 they got together and said “guys you know what would fuckin hilarious?” lmao but the parties did fully switch positions with regard to who were the progressive reformers and who were the conservatives, just objectively.
Idk who downvoted this but it was literally just true mid-party switch that the divides were more regional than partisan. Northern Republicans AND Northern Democrats supported the Civil Rights Act, Southern Republicans AND Southern Democrats opposed it. Then in 1964 Barry Goldwater ran for President on an anti-Civil Rights platform, solidifying the current state of the two-party system and officially making race relations a partisan issue.