The CSA wins the Civil War, for whatever reason - it doesn't really matter, honestly. It was an extremely narrow victory, with Northern manpower and industry making the fight incredibly hard. They come out of the war with only the main CSA states, Kentucky, and Oklahoma. The CSA recognises the need for more industry, and begins a crash course in industrialisation. But why should the white man work in the factories when we've got all these slaves, someone says - and so slavery begins the transition from agricultural to industrial. The Union hears of the boll weevil in Mexico a bit earlier and decides purposely spreading it would be good revenge against the CSA - the weevils are intentionally spread across the South as fast as possible, causing cotton to collapse. To the North's surprise, the Southern plantation owners don't panic as much as expected, instead making slavery over 75% industrial by 1900. The Great Migration northwards never happens, so the CSA has more slaves than the OTL South, and the North has far far less African Americans.
Socialism/Communism are invented mostly as IOTL, and is smuggled into the CSA's slave factories, gaining immense traction there. This American Communism is more democratic than Soviet Communism, and has intensely strong demands for equality. The ever-present slave revolts become more organised and more insidious approaching the 1920s. The CSA is already in chaos, but when some unspecified economic disaster similar to the Great Depression hits the CSA, the spark of revolution lights. Many poorer white southerners are impoverished by the crisis as well, while black southerners are treated harsher and harsher in order to boost production and to stop the revolts. This backfires, further radicalising the slaves, while the economic situation improves for no one but the rich. Something eventually breaks, and a massive wave of slave factory revolts spreads across the country, waving the red flag of communism. This revolt is far larger than any other, hitting nearly every factory in the Black Belt almost simultaneously - it's a true revolution. White communists are conflicted, but after support from prominent leaders, enough of them join the revolution that it can be said both races are working together. Oklahoma's natives also join the revolution, mainly just to escape the racist CSA government.
The entire Black Belt is red - the CSA government evacuated Richmond to Kentucky, mostly safe from the revolution. But they can't hold for long. Only Texas, Kentucky, Tennessee and Florida aren't being attacked by revolutionaries, and Mississippi, South Carolina, and Alabama are under total Red control. Florida falls, isolated from the rest, and the CSA loyalists (known as the Whites for a different reason) put up a good fight in the Upper South, but are being pushed back. Texas, which had always been agitating for more States Rights in the CSA anyway, declares independence and signs a ceasefire with the revolutionaries. White power is finally shattered at the Battle of Louisville, with red militias scoring a decisive victory. Retreating White forces run North to the USA - some are arrested, others get asylum, but all spread exaggerated tales of "red negroes raping and murdering our families" - in reality, there was looting, but it was no worse than what White forces had done to revolutionary towns before. The US public is nevertheless shocked, and public opinion of the revolutionaries slowly begins turning from "brave freedom fighters against the tyrannical slavers" (hence the previous non-interventionism) to "uncivilized, barbaric monsters that want to kill all whites", thanks to media moguls fearing communism and racists fearing African Americans. This is despite a decent number of revolutionaries being white themselves.
The CSA dies, and the Southern Communist Republic is born as a democratic (read: left parties only) multi-party state. White slaver refugees filter North, carrying more horror stories of black communist militias killing whites for their farmland and property spread from the South (some are true, many lack context, and some are outright fabrications) stirring up racist sentiment. Down south, the SCR heavily enforces racial integration and punishes racism with severe penalties. Many white revolutionaries feared they'd become persecuted or even enslaved, leading to a policy that hate crime judgement would require white and black judges to come to consensus (and that these cases would set legal precedent should the races be reversed in a subsequent case). The North becomes more and more racist over the 30s, and drifts further right into a cross of corporatism and white ethno-nationalism due to the revolution next door. Meanwhile, Southern integration policies initially cause resentment, but substantially decrease racism as time goes on.
By 1940ish, the SCR has recovered from the Civil War, gone through their version of Reconstruction, and comprehensively purged slavery from society. With the North making threatening, revanchist noises, the USSR or whatever other major Communist power sends substantial military aid to help the SCR military, deterring the USA enough to survive. The USA has fallen to a quiet, long-term Business Plot equivalent - you can vote for your candidate, but both have been vetted by leading companies as being sufficiently right-wing and anti-SCR (and thus generally heavily racist). Propaganda is everywhere, rallying against the SCR's degeneracy. A white ethnostate party exists and has seats in Congress. Texas and Mexico are sweating bullets and end up falling into the British sphere.
By 1960ish, assuming neither side was willing to go to war, the SCR is a stable-ish Communist Democracy that has its problems but is generally a pretty decent place to live. Racism by now is unacceptable and a fringe notion. The USA is still a "democracy", but corporations dug their hooks in further and further over the years. Most blacks were kicked southwards into the SCR, and only western European immigrants are allowed.
By 2000ish, the SCR's main backer has likely fallen, while the SCR itself is maybe still going along OK if it managed to reform properly from pure Communism. Racism is almost entirely gone (although a lot of this is because hate crimes are treated far worse than regular crimes - the SCR will only give the death penalty for high treason or lynching) and the nation has had black, white, and mixed leaders without much issue. Had it collapsed, it has reformed into a democracy but the previous mostly still applies. The USA is either basically as it was before, or has collapsed into numerous states. Some might be progressive-ish, one is probably Nazi-esque, the rest are somewhere in between. A lot of these successors are still horribly racist, they just try to hide it from the public. Luckily, they don't have anyone to really be racist to.
