If you ask them, they'll say he's not electable. They've felt this way from the beginning, and backed Jeb Bush instead.
Some Republican stalwarts have also said Trump isn't a "true conservative" because in the past he's supported abortion and a national health care plan. But Trump is in fact more conservative on the issues which have resonated with voters than the Republican establishment has been.
Trump has spoken out against illegal immigration more strongly than any other candidate. This makes the establishment uncomfortable because they are terrified of being accused of racism. But turning a blind eye to illegal immigration helps also big business by driving down their labor costs.
This is the issue on which their -- i.e., their donors' -- interests are most directly aligned against the middle class.
Big business also likes to farm plants out to India, China, the Philippines, and Mexico without penalty. They also like being able to reincorporate in Ireland and other tax havens.
Trump has spoken out against these practices. He has also suggested that hedge fund managers out not to have their management earnings taxed at the long term capital gains rate. (Hint as to where the Republican establishment stands on this issue: hedgies donate big bucks to the GOP.)
There's also the discomfort caused by Trump's criticizing of Republican orthodoxy. But who has more credibility: Trump, for saying that the Iraq War was a mistake, or the Republican Party, for refusing to acknowledge that?
The Republican establishment, naturally, found this embarrassing. But polls show that a majority of Republicans and plurality of all voters agree with Trump about Muslim immigration.
Trump has also denounced the soft corruption of campaign contributions. This scares not only the big business donors, but also all the Senators and Congressmen hoping to eventually make a handsome living on K Street.
The Republican Party has always paid lip service to the middle class, but has enacted policies which favor their rich donors. And that's where Trump really scares them. If they can't control him, donors would have less reason to give the Party money. Even worse, Trump might try to reform that system.
It is true that Trump has turned every criticism he's received into a personal vendetta, and part of the job description for the Presidency is "thick skin." Calling Carly Fiorina ugly was ugly, saying Megan Kelly was "bleeding from her wherever" was a poor choice of words, and saying John McCain wasn't a war hero was ridiculous. But so far, none of the salvos from this loose cannon have backfired.
Yes, Trump is a bully and an egomaniac. But that's not why the Republican establishment fears him. The real reason is, they want to keep their donors happy.
Otherwise, their spigot gets turned off, and then who would support them?