Many Republicans oppose Trump because they see him as the one candidate most likely to lose to Hillary Clinton. But others fear something worse: a Trump victory. They know that his presidency would represent a grave danger to the republic, a repudiation of the most noble Republican aspirations, and the end of their party as a serious vehicle for governance. The GOP can survive a Trump defeat. It will never get over being permanently defined by his politics of flippant brutality.
You know, they haven't actually been hiding it. Even that nice Mr. Brooks isn't, you know, nice. Respectable demeanor is not the same thing as nice. They've been all about kicking the poors and the blahs and the gays and the whoever they can get the press to see as a "sides disagree" issue (are black people stupid? the two sides disagree) that week as long as I can remember. Trump just isn't as well-trained and he does not give a fuck. There are plenty of reasons to dislike Trump, but most of those are reasons to dislike most Republicans. With Trump, the people who think they should and do run the country worry they'll lose control. That's the main difference.
It isn't a reason to vote for him, but it's way down the list of reasons to vote against him.