A couple thoughts on the change of political parties...
After the Civil War the southern states were nearly 100% Democratic. From the governor down to the dog catcher, you almost couldn't get elected as a Republican. The familiar liberal/conservative lines still existed, but it was essentially one party rule. On a national level, this meant that many southern Democratic senators and congressmen could be, and often were just as conservative than typical Republicans.
This started to change in the 1970s with Nixon's "Southern Strategy" which courted conservative southern Democrats, and started to take off in the 1980s with Reagan. Two things really sealed the deal, one was the Democrats losing the House majority in 1994 at which point many remaining southern Democrats decided to switch parties, and the second was courting evangelical voters, who vote reliably anti-abortion. Abortion is a classic wedge issue where people who feel strongly about that issue will vote regardless of other factors. This lead to the near death of the NE liberal Republican. George H.W. Bush for example was pro-choice, and Mitt Romney ran for senate as a pro-choice candidate.
Enter Donald Trump. Trump keeps the abortion issue, along low taxes, and a few other things, but otherwise flips the script. Trump is in favor of tariffs and a generally isolationist viewpoint, which appeals enough to white working class voters who had formerly mostly been Democrats, and wins the election.
The Post-Trump Republican party faces some big questions. The economically liberal (in the classic sense) wing of the Republican Party isn't compatible with the Trump wing. The aggressive foreign policy wing isn't either. I don't know how this plays out, but a different Republican Party will emerge from the post-Trump era. And a different Democratic party will too.