In this essay, Neal Pollack explains what made him switch.

So it is with the utmost intellectual and moral authority that I can state that Donald Trump represents the greatest threat to the Republic’s moral standing since the rise of pay-per-view motel porn. While we managed to endure that other scourge, and even thrive with it, my considered opinion is that Trump would finish us off. And not in a porn kind of way.

Mr. Trump lacks the character, values, temperament, experience, testicles, or, most importantly, hairstyle to be President. He appears to also lack basic knowledge of the workings of government, of the secret workings of government, of the U.S. Constitution, of the Secret U.S. Constitution, and of basic U.S. institutions like freedom of religion, freedom of expression, and an independent judiciary that you only see on weekends at the racquet club.
…

So now I must join my fellow Princes Of Darkness (as we affectionately call ourselves at the Bohemian Grove and the other, secret Bohemian Grove that no one knows about) in bravely condemning the greatest threat conservatism has known in our lifetimes, and in all other lifetimes in all other dimensions. Just the other week, I had catered lunch with John Bolton, G. Gordon Liddy, Lex Luthor, Skeletor, Henry Kissinger, Count Dracula, Baron Zemo, Karl Rove, Roger Ailes, and the surviving commanders from the My Lai Massacre. We all agreed that Trump is the worst man ever.
…

Trump is not the kind of person you would want in charge of your nuclear arsenal, or of any other kind of arsenal, for that matter. My friends and I spent a lot of time setting up this world order. If a madman upsets it, it’s going to be a madman of our choosing, at a time in history that suits our purposes.