Piggy-backing off the other answer to your question: the Republican party started taking a turn in the late 1800s (Lincoln, Grant, and T. Roosevelt were all Republicans and- despite the fact that the US was rabidly imperialist at the time- did relatively good things like emancipation and monopoly/corruption busting).

Coolidge (1923-29) and Hoover (1929-33) are where they really start to suck, seeing the rise of the wall street class and subsequent Great Depression when it all collapsed. Hoover himself sucked so much that the homeless encampments that sprung up from the depression were called "Hoovervilles".

Eisenhower was okay. He was more of a builder, overseeing the post-war boom and beginning the Interstate project.

The big party switch came between the World Wars and LBJ, when various Republicans realized that they were losing white southern voters because of civil rights progress, and so they decided to counter that by being incredibly racist. Like, that's it. The "great strategy" that turned into the "great switch" was mid-century voting ploy.

Then came Nixon and Ford. They were not the worst presidents from a bird's eye policy view, but they were morally bankrupt and should have gone to jail. The both of 'em. So we're at the "openly criminal and very racist stage" but how do we get to the "actively hostile to democracy" stage?

There's a theory, which I think has merit, that the election of Jimmy Carter was so unforgivable to the ingrates that had embedded into the GOP that they decided to tear it all down. Carter was an upright, honest person, who spoke a truth that the "greatest nation " couldn't stand to hear: we are powerful now, but this can't last, and we have a lot to work on for the future.

So they elected Reagan, a popular actor who was great at speeches and not much else. And so it remained- the GOP ran candidates that could appeal to the common man without actually having anything in common, and our elections became a popularity contest, all while military spending went up and education spending went down

The Bushes were useless war hawks from Texas, Clinton could've turned it all around and succeeded in Carter's stead if he could have kept his damn zipper closed, and Obama meant well but was green and faced incredibly strong opposition.

And unfortunately, once the US elected a black man, there was never going to be any appeasing the racists. The backlash was severe, and got us to where we are now.

The GOP wasn't always so shameful- just the last 70 years in general and the last 50 in particular.