To answer the original question, the most immediate cause was the Republicans' loss of the West Coast, especially California. There have been two CA governors who went on to be president: Nixon and Reagan. Orange County, CA was basically the headquarters of American conservatism (note: the Claremont Institute is located there).
Look at the 1976 presidential map. It's close to the opposite of our maps today. When Pubs lost California, having already lost NY and then-populous Michigan, their only path to national relevance was picking off the southern states. And the southern states were never conservative. They were just racist, and happy to throw in with whomever would let them discriminate, or if they couldn't, at least feel good about wanting to.
Orange County was all about the classic Eisenhower-Reagan conservatism. California was dominated by the military industrial complex; they wanted a strong foreign policy (and the demand for ships and aircraft that came with it). It was an exporting state, so it favored free trade. It was rich, so it liked the trickle down economics. And that was the ideology that dominated the Republican party.
So what happened in California? Well, I'm far from an expert on CA history; and even if I was, I couldn't encapsulate everything that happened in a message board post. I can say with confidence, though, that much of it was about brown people. For thirty years, they had been bringing Mexican farm workers into the US on guest worker programs (e.g. bracero), and those Mexicans had babies; and when the babies grew up, they started to fight back against the anti-immigrant, racist sentiment in Orange County. And California also had a great university system, and a high % of people attending college, and we know what happens when people attend great universities. They tend to lose their prejudices.
It's no coincidence that the idea of reinterpreting the 14th Amendment to exclude birthright citizenship was developed and championed by the Claremont Institute (which is why I mentioned it above). Figures associated with Claremont include John Eastman and Michael Anton. They understood that birthright citizenship was a threat to their particular political program, so instead of modifying their politics, they went after birthright citizenship. But there was no market for that shit in CA any more. They had to find more fertile ground. You know, the south.
Again, southern states were never conservative. White southerners were never conservative. Hell, FDR's base was the Solid South. White southerners who suffered during the Depression were eager for Social Security and Medicare (note: it was Reagan in the 1960s, not any southern governor, who called Medicare socialist). But just like LBJ said, you can get a white southerner to sign onto anything if it makes them feel better than black people. And that's pretty much it. All that shit that the Southerners claim to value today -- anti-abortion, Second Amendment, etc. -- has never been more than a rationalization, when in fact the problem is that they just didn't want to be in a political coalition with black people.
True story: in the 1960s, the NRA was pro-gun control. It wasn't until the 1970s that the Second Amendment bullshit got started. And who started it? Some dude from Texas who killed a Mexican as a teenager, but escaped prosecution because it was the 30s or 40s. The Southern Baptists were pro-abortion -- until, as many other posters have observed, the feds came for Bob Jones U (and in the West, BYU). All roads in Southern politics point back to racism.
Hell, even the existence of Southern politics is about the racism. What the fuck do Texas and South Carolina have in common? Why would they vote in lockstep pretty much all the time? The demographics are different. The economies are very different. The economic structures are different. And Missouri and Arkansas are different from both. And yet, these states have ALWAYS been in a political coalition. They were rebels together. They were New Dealers together. And now they are GOP/MAGA together. The common thread is the racism. That's what it has been about for 200 years.
And thus we get MAGA. They were never interested in the Orange Country Republicanism. What happened was that Orange County Republicanism died with the Orange County Republican party (I believe HRC was the first Dem to win Orange County, and it's remained Dem ever since). What remained was the exclusionary populism.
