Many former Trump aides say he shouldn’t be president. Will it matter?

John F. Kelly, the longest-serving chief of staff in President Donald Trump’s White House, watches Trump dominate the GOP primary with increasing despair.

“What’s going on in the country that a single person thinks this guy would still be a good president when he’s said the things he’s said and done the things he’s done?” Kelly said in a recent interview. “It’s beyond my comprehension he has the support he has.”

Kelly, a retired four-star general, said he didn’t know what to do — or what he could do —...