There are broad features of the parties that haven't changed that are necessary to understand the timeline of southern realignment.

Republicans had been trying to win southern whites since the turn of the 20th century. That caused a large number of African Americans to abandon the party. They started voting overwhelmingly Democrat in 1936.

But the Democratic Party didn't support the Civil Rights Act until 1964. And the south remained solidly Democratic well into the 1980s, outside of Presidential contests. Many completely white rural southern counties voted for Carter and Clinton. What explains that timeline?

You have to look at the economics and the law. FDR's Democratic Party was recognizably modern: technocratic, and proponents of the administrative state, regulation, and economic redistribution. Southern whites stayed in this coalition because the south was poor and agrarian, and had interests averse to those of northern businesses. New Deal programs were instrumental in industrializing the southern economy: https://siepr.stanford.edu/sites/default/files/publications/.... In 1930, the southeast was the poorest region, with half the national average income. By 1970, the gap had closed dramatically, to 80%. https://www.visualcapitalist.com/comparing-wealth-u-s-geogra...

The Republican Party, meanwhile, continued to be a small-government, anti-regulation party during this time. Goldwater tried to win the south by opposing the Civil Rights Act of 1964 on small-government grounds, and while that worked in a few deep-south states, he lost every normally Republican state except Arizona.

By the time Reagan came along, a couple of things had changed:

1) The south's economy had dramatically industrialized, creating prosperous suburbs of the kind Reagan won all over the country.

2) The legal debate over the Civil Rights Act had long since been replaced by debates over affirmative action. The 1980 Republican Platform reflects this:

https://www.presidency.ucsb.edu/documents/republican-party-p... ("However, equal opportunity should not be jeopardized by bureaucratic regulations and decisions which rely on quotas, ratios, and numerical requirements to exclude some individuals in favor of others, thereby rendering such regulations and decisions inherently discriminatory.").

With the illegality of de jure discrimination settled by Brown v. Board, and commercial discrimination settled by the Civil Rights Act of 1964, the debate shifted to addressing what we'd call today "structural racism." And on that point, the Reagan platform reflects the same limited-government Republican ideology that existed since before Hoover. And Democrats' support for government intervention and affirmative action reflected their broader 20th century ideology as well.

FWIW, I happen to side with Democrats on this one, at least with respect to economic interventions. But the parties have more or less the positions you’d expect ideologically.

There is a good book that covers this: https://press.princeton.edu/books/paperback/9780691133898/th.... It gives a similar but slightly more nuanced take than Pearlstein, which I don't think is wrong, just incomplete and a bit misleading.