Authoritative and measured, Mr. McCullough typically strikes a tone of determined neutrality. In public appearances, he said, he deliberately avoids commentary on contemporary politics.

“Very often, during question-and-answer sessions, people ask me some question about the president or other would-be candidates,” he said in an interview this week. “I’ve always said, ‘My specialty is dead politicians.’ In that way, I could sidestep the question without getting myself involved.

“But this time around, I don’t feel that way any more.”…

Mr. McCullough, raised in a Republican home and now aligned with no party, said the prospect of a Trump presidency so distressed him that he felt he could not remain publicly detached. “When you think of how far we have come, and at what cost, and with what faith, to just turn it all over to this monstrous clown with a monstrous ego, with no experience, never served his country in any way — it’s just crazy,” he said. “We can’t stand by and let it happen. The Republican Party shouldn’t stand by and let it happen.”