One of the fictions of the 2016 campaign was the belief that the GOP Establishment somehow wished for Hillary Clinton to win, somehow, as long as they could (A) not be blamed for her victory and (B) separate the Presidential campaign from the downballot races. This just wasn't the case. There were more renegade GOP elected officials refusing to endorse the national ticket in 2016 than those who bailed on Goldwater in 1964. More Republican elected officials conspicuously bailed on Trump than Democratic elected officials on McGovern.
The GOP Establishment wanted Trump to win, if only because they viewed him as a Signer-in-Chief who'd sign their bills, and that hasn't changed. They have come to accept that 2/3 of a Republican is better than no Republican at all. They are also aware of the fate of political parties that challenge their incumbent Presidents; iit's a no-win situation. One has to go back to 1856, when James Buchanan challenged Franklin Pierce did a party maintain the White House after their President was significantly challenged for renomination. They're not going to let this happen; any challenge to Trump's renomination will go no further than Pat Buchanan's challenge to Bush 41.
Or he could continue the entirety of Jimmy Carter as he has done magnificently well at, even at this point. Even down to getting his own Ted Kennedy.
You also conveniently forgot about Reagan's challenge to Ford in '76. True
that much vitriol is unlikely, but the point still stands.
