I think when the choice becomes Trump or Clinton, he will back Trump. He won't campaign for Trump or anything like that, the endorsement will be a formality.
I was starting to think, of course he would. But stuff is probably getting personal enough, that he won't. The question is, then, what does he do? He'd never endorse Hillary outright. But would he give in on his treasured principles? Would he actually hate Trump enough to give a wink wink Hillary endorsement? Would he just keep his mouth shut (thats hard to imagine)? I guess he probably would, because most of his supporters will.
Christine Todd Whitman said that she'll probably vote for Clinton if Trump is the GOP nominee, but I can't really see any Republican politicians who currently hold office endorsing her. All of the "Trump skeptics" in the GOP who currently hold office will do one of the following:
1) Say that they're going to vote third party.
2) Say that they're going to write in a name. (Likely a totally symbolic name--someone who has no interest in running. E.g., in 2004, Lincoln Chafee said he wrote in George H.W. Bush's name rather than vote for Bush II or John Kerry.)
3) Say that they'll grudgingly vote for Trump solely because the alternative is worse, not because they think he'd be a good president. ("Endorsement at a distance", so to speak.) They'll keep Trump at arm's length, and won't do any public appearances with him.