Perhaps if the Dems turn to the right on social issues, make the economy truly progressive and the New Deal coalition is retained and experience a far-reaching Revolution on the scale of OTL Reagan's in the chaos of the 1980 election, and the South stays Democratic.
The Republicans are the ones signing Civil Rights, and upscale blacks become their voting bloc, while downscale blacks remain Democratic. This Republican Party is environmentalist, Keynesian, supports New Deal programs, is socially moderate-to-liberal, basically the Rockefeller Republican. They retain their support for the New Deal given the NDC's vindication in the 1980s and don't implement neoliberalism.
How would California vote in this scenario?
P.S. Obviously, this GOP won't support Prop 187.