It seems likely that if Trump loses in 2020, all bets are off as to what the Republican party does. It's absolutely been imploding for quite some time, since the Tea Party movement and Trump splitting the party as well. It certainly feels like Trump losing in 2020 results in a dramatically changed Republican party. It feels inevitable to fall apart in either 2020 (with a Trump loss) or 2024 (if Trump wins).
I'm not sure about that, tbh. I mean, yes, Trump is a disaster for them in some ways. But the thing is, he definitely has galvanized voters in a remarkable way, and made them incredibly loyal to him. I'd argue that his particular pathological narcissism has allowed him to stumble across a winning formula. He's just bad at it (no impulse control, actually not smart or competent or qualified, more of an idiot savant).
My gut feeling is that the Republican party, once they pick themselves up and brush themselves off (assuming a Trump defeat, which I still don't want to think of as a given), will take the following lesson: do what he did, but just better. Less stupidly. Just as brazenly lying with a straight face, just as blatantly ignoring the Constitution, but just doing it with a little more finesse, and maybe not
quite so much literal palling around with dictators of hostile nations. That could be a winning formula for them in 2024.
When GWBush was elected, I thought, "These Republican nominees just keep getting worse lately. Wow, wonder how the next one could be worse." (Arguably, McCain and Romney were better, but they didn't win, now did they?) Now, since Trump has been in office, I have been wondering the same thing about the next GOP nominee. My verdict: As corrupt as Trump, but much cleverer.