NY Times has an article on the Republican Leadership. To sum up: The republican leadership is now quietly panicking at the idea of a Trump as Nominee, not only for handing the Presidency away, but potentially giving away the senate as well.
Another Republican strategist in Ohio replied to an email asking about Mr. Trump’s effect in the state by sending a link to a Wikipedia page on the 1964 congressional elections, when Barry Goldwater’s presence atop the Republican ticket led the party to lose 36 House seats.
As for my own analysis, it's hilarious. There is NOTHING they can do. The entire system that they in some cases themselves built is entirely rigged towards making it impossible to stop him. They can't make him quiet. They can't exclude him. They can't starve him of cash. They can't coalesce behind anyone.
I think they are just afraid to, honestly. One of the reasons he is still up is no one, really strong candidate or group has just stood up, said "enough is enough", and destroyed Trump. The only thing that could chastise him would be a sufficiently powerful support base, political campaign, and media machine, and no Republican has been willing so far to risk dive-bombing Trump, since it doesn't seem to go so well for those who have tried partially (who attacked Trump? people low in the polls now. think about it). If Trump really faced a coordinated campaign to discredit him, then, finally, could the sheer amount of evidence of his personal failings be really used effectively.
Thus, the best part of a Trump ticket would be the ENTIRE Democratic Party Apparatus, from national to state to local, turning into a "Death to Trump" rally. They would try and link every single republican who exists to him. In more conservative regions, that might even help, but any place with moderates will just abort en masse. There just exist so many things to discredit him with: flip-flopper, liar, thief, crook, .00001%, bigot, RINO, crypto-liberal, proto-fascist, reactionary, moron, loser, failure, big-wig, clown, bully, etc.; and that isn't even a distorted list, you could legitimately argue every single one of those things (not necessarily true, but serious arguments). There's just no thing he isn't that people don't like, and there's plenty more you could pretend him to be, if only enough effort was put into it. Half-measures won't bring him down. Either he comes down on his own, or in glorious free-fall.