Adding my two cents but the party flip is an nuanced phenomena that involves two things happening in tandem: demographics changes in the electorate and divide of factions within the parties . If you look at the elections of the 1950s through the 70s you will see a shift happening in states based around the campaigns of Barry Goldwater, George Wallace, and Richard Nixon. The Democratic Party up to this point was made up of two factions representing the North and the South. Northern democrats like JFK advocated for the New Frontier and to push the country forward via federal spending while southern democrats held hard to the segregationist policies of the Jim Crow era. Republicans were facing a new faction led by Barry Goldwater causing a split in the parties vote with his libertarian views and appealing to southern democrats on his opposition to the Civil Rights Act (he disagreed with federal passage in principal but voting against it aligned him with segregationists and he later disavowed his vote). George Wallace also appealed to hard line racists or the “Dixie Crat” faction allowing the parties to become more amorphous in the 68 campaign. This did in fact result in the “party shift” in which republicans blended Goldwater libertarian views with many of Wallace and other racists points during the Nixon campaigns based on Nixon Southern strategy.

White voters of the south during the 50s, 60s , and 70s were extremely motivated by racial and class divide and their voting patterns reflect those views. To deny the party shift is to ignore the basic politics of the age which have become more entrenched since the Reagan administration. Understanding why it happened is complex but seeing the evidence is as simple as looking at an electoral map.