From his latest New York Times column:

The Republican Party, under Trump, is headed for a combustive breakup. Ronald Reagan envisioned a shining city on a hill. Trump represents nothing more than a thug with money. Can the man who shouts “I’d like to punch him in face!” and “Knock the crap out of them!” about protesters possibly quote Abraham Lincoln, Teddy Roosevelt, Dwight Eisenhower or Reagan in his acceptance speech? He could, but it would be no more plausible than a fish doing a tap dance.

He concludes:

The choice for honorable Republicans — should I stay or should I go? — is obvious, though not easy.

More here.