The Todd family gave up editorial control of Abraham Lincoln's papers around 1947 or so, and the new scholarship of the 1950's proved the Real Lincoln was not even remotely their 'friend', new books were written, the GOP imagery was lessened, and another factor was the invention of mechanical cotton pickers in the late 1940's leaving a lot of black people unemployed.
Republicans had long taken their votes for granted and offered them nothing, so they shifted to Democrats and independents, like George Wallace to name one, in the 1950's. JB already explained the shifts in northern black votes, via the Roosevelts dominating the Party. The rest after that is very easy to follow, except for those who refuse to read history and prefer ideological nonsense.
Nixon began trying to win them back almost the very day he was first inaugurated, reviving the defunct quota system over affirmative action, defeated just weeks before by Moynihan and others, and also began the 'War On Drugs' in black communities, at the behest of Charles Rangel and the Black Caucus. It was too late, as LBJ and Sam Rayburn had already beat them by the time of Rayburn's death in 1962 or so, via the Civil Rights Acts and other legislation.
And to this day yankees and neo-liberal vermin can't stand the fact that it took 'southerners' like LBJ to get it done, while their darlings the Kennedys and all the other northeastern Democrats never got squat done, hence the Camelot Myth and the incessant LBJ bashing spewing from academia and the northeastern pseudo-liberal establishment ever since. The South adapted much better to integration than the North did, another embarrassment heaped on yankees by those 'ignorant rednecks' down south as well. See the school busing wars in the north during 1970's and 80's that re-segregated northern schools to levels the South hadn't seen in decades. lol
Real history is messy, and almost everybody who attempts to try and use it to make ideological points is going to be disappointed and end up relying on selective cherry picking gibberish in trying to force it to conform to some trope or other.
