It's not unthinkable that a decent number of Republicans would flip, but a precondition of that would probably need to be a plummeting approval rating for Trump

It's not unthinkable that a decent number of Republicans would flip, but a precondition of that would probably need to be a plummeting approval rating for Trump, which hasn't borne out so far - despite all the controversy, he still seems to have a 35-40% floor. One interesting thing about Trump is that he has some of the most stable (albeit somewhat low) approval ratings of any president that there's reliable data for. But he's gone from "historically unpopular" to "about as popular as Reagan, Ford or Clinton" at this point in his presidency.