Peter Wehner of the Ethnics and Public Policy Center has a strong op ed summarizing the reasons why his fellow conservative Republicans should oppose Donald Trump:

Beginning with Ronald Reagan, I have voted Republican in every presidential election since I first became eligible to vote in 1980. I worked in the Reagan and George H. W. Bush administrations and in the White House for George W. Bush as a speechwriter and adviser. I have also worked for Republican presidential campaigns, although not this time around.
Despite this history, and in important ways because of it, I will not vote for Donald Trump if he wins the Republican nomination….
There are many reasons to abstain from voting for Mr. Trump if he is nominated, starting with the fact that he would be the most unqualified president in American history….
During the course of this campaign he has repeatedly revealed his ignorance on basic matters of national interest…
No major presidential candidate has ever been quite as disdainful of knowledge, as indifferent to facts, as untroubled by his benightedness.
It is little surprise, then, that many of Mr. Trump’s most celebrated pronouncements and promises — to quickly and “humanely” expel 11 million illegal immigrants, to force Mexico to pay for the wall he will build on our southern border, to defeat the Islamic State “very quickly” while as a bonus taking its oil, to bar Muslims from immigrating to the United States — are nativistic pipe dreams and public relations stunts….
For Republicans, there is an additional reason not to vote for Mr. Trump. His nomination would pose a profound threat to the Republican Party and conservatism, in ways that Hillary Clinton never could. For while Mrs. Clinton could inflict a defeat on the Republican Party, she could not redefine it. But Mr. Trump, if he were the Republican nominee, would…
Mr. Trump is precisely the kind of man our system of government was designed to avoid, the type of leader our founders feared — a demagogic figure who does not view himself as part of our constitutional system but rather as an alternative to it.

Other leading conservatives have also pointed out the grave flaws in Trump’s platform, qualifications, and temperament, including Charles Cooke and George Will. But Wehner does a particularly good job of hitting key points. I would only add that Trump’s disdain for constitutional property rights and his long history of crony capitalism should also trouble conservatives, or indeed free market advocates of any kind.

Peter Wehner was my supervisor back when I was a college student intern at Empower America, in the summer of 1993, working for the late Jack Kemp and others. He is far more conservative than I am, especially on social issues. But we are in complete agreement on Trump and the danger he represents. I too will not vote for Trump if he wins the GOP nomination, and agree that Hillary Clinton is a much lesser evil. We should put principle ahead of partisan bias. But such a statement means more coming from a committed Republican like Wehner than it would from me.