It definitely slows the realignment of American politics along ideological lines. At the same time it increases pressure on the coalitions in each party.
The Democrats were ready to pop and had honestly only been sustained by a series of individual relationships and ever-diminishing post-Reconstruction momentum. That coalition was going to unravel. If their share of the black vote is smaller, that means the Dixiecrat share is larger, and that's not something non-Dixiecrat Democrats could really tolerate. It's one thing to pay lip service to southern whites, to turn a blind eye to their worst behavior, or to create a narrative where they're evolving into a new south (i.e. the Carter model), it's another thing if they hold the balance of power in your party between labor and urban liberals.
IOTL the shift to more minority voters and more middle class voters made the passing of the Dixiecrats an acceptable trade-off (especially since the timescale on that transition was decades long). Here, that's not happening, and with Dixiecrats feeling more empowered but probably less welcome in both parties, there's a reckoning coming soon. At "best" you might get a situation like we saw recently in the New York state house, where a group runs nominally as Democrats but is willing to caucus with whichever party gives them the most. At "worst" the two-party system faces a big challenge. (Really at best, they take their ball and go home, forming a third party that fares about as well as all third parties do in the US, takes millions of culturally conservative votes out of the system, and results in a few decades of unimpeded progress on the social policy front as Dems and Reps suddenly don't have to pander to that demographic anymore.)
The Republicans are more of an unknown. I'm inclined to say that the GOP did a better job keeping its factions in line during this period, but the truth is they were really only ever like 75% of a party from the Depression until the 1990s, capable of winning the presidency but rarely controlling even one chamber of congress. Being in the minority has a way of muting factionalism, outside of key moments at least.
My inclination is to assume that Calcaterra is onto something with that One Nation Conservatism idea. The African American community is as ideologically diverse as any segment of the population. Historically, the Democrats emerged with a narrative of social liberalism and economic moderation, and so the African American politicians of the party tended to follow that line, though the communities they represented were (and are) not totally unified around those principles. African American politicians of an alt-GOP would likely do something similar: tacking to the broadest point in the party's platform and staking out ground there, though the communities they represent will not be totally unified around those principles. But at the same time the conservative factions will have to give some ground in the name of electoral pragmatism. (Of course I say that and the evidence is all around me that it isn't so- the new Republican is "supposedly" a poor working class white man, and yet conservative think-tanks and chief policy architects aren't really making policy concessions for them. But the idea is that movement conservatism would've been more pliable before it totally took the reigns of power in the party in 1980.)
A GOP that hovers over the middle ground could really break the Democrats. My feeling is that it pushes the country to the left because it keeps working class farmer/labor whites from turning into Reagan voters and we get more generations of left-leaning politicians emerging from a broader set of congressional districts, while the emerging service economy unavoidably pushes college education, which naturally pushes more leftist behavior in major metros.