You seemed to have left out the Southern Strategy and the War on Drugs, which went a long way towards splitting the parties.
Conveniently towards the end of your timeline, Nixon during his 1968 campaign appealed to many dixiecrats who were upset about the passage of the civil rights act, stating it was a form of government encroachment on their lives. This in turn led to a party shift where former dixiecrats turned republicans and in time, the former republicans turned progressive. Once the parties flipped, you've got the War on Drugs for the 1970s and 1980s that heavily criminalized communities of color and anti-war liberals. Or as John Erlichman said it "We knew we couldn't make it illegal to be either against the war or black, but by getting the public to associate the hippies with marijuana and blacks with heroin, and then criminalizing both heavily, we could disrupt those communities. We could arrest their leaders, raid their homes, break up their meetings, and vilify them night after night on the evening news,"
Blacks, who were outright demonized by the right, turned to the left, which accepted them because who would not turn down the free vote. The left still treats black voters in a passive-agressive manner, knowing they can reliably count on the black vote, but the relationship is not as antagonistic as what is seen on the right.
So many right wing people love to crow about how Republicans freed the slaves and were responsible for most progressive legislation early on in this country. All that is true, but the conveniently leave out the part where Nixon and Lee Atwater flipped the parties, and when both Reagan and Bush used race based fear mongering to further drive the republican base whiter and more conservative.