The way the Democrats are working themselves into a lather over whether President Trump will commit himself to a “peaceful transferal” of power reminds us of the moment when we first realized the New Yorker could win the 2016 election. That was eight minutes into the first Republican primary debate, when moderator Bret Baier asked whether they would all pledge to support whoever emerged as the GOP nominee.

Here’s how Mr. Baier put it: “Is there anyone on stage — and can I see hands — who is unwilling tonight to pledge your support to the eventual nominee of the Republican Party and pledge to not run an independent campaign against that person?” There were ten candidates on the stage, and every hand went up, save for that of Mr. Trump. The savvy Mr. Baier no doubt knew he’d opened up an illuminating moment.

“Mr. Trump to be clear,” Mr. Baier said. “You’re standing on a Republican primary debate stage.”

“I fully understand,” Mr. Trump replied.

“The place,” Mr. Baier said, “where the RNC will give the nominee the nod.”

“I fully understand,” Mr. Trump said.

“And,” Mr. Baier plunged on, “that experts say an independent run would almost certainly hand the race over to Democrats and likely another Clinton. You can’t say tonight that you can make that pledge?”

“I cannot say,” Mr. Trump replied.

What that moment did, in our estimate, was mark Mr. Trump as presidential timber — as an individual who could stand apart from the crowd. He could easily have pledged to support whoever won the primary. Yet he knew that he was being asked to give away an option, something Presidents are importuned to do all the time. And that the others, by agreeing to it without conditions, could be seen as shortsighted and even weak.

And, in the case of some of them not even honest. Governor Kasich did not support the Republican nominee, and some of the others did so only half-heartedly. Once the two parties chose their nominees, a new round of debates began. In the first, at Las Vegas, moderator Chris Wallace said: “I want to ask you here on the stage tonight, do you make the same commitment that you’ll absolutely accept the result of the election?”

Mr. Trump hung back. “I will look at it at the time. I’m not looking at anything now, I’ll look at it at the time. What I’ve seen, what I’ve seen, is so bad.” He then attacked the press for having “poisoned the minds of the voters.” He proceeded to attack the press as “so dishonest and so corrupt.” Then he predicted the voters would see through it, and noted that “we’ll find out on November 8th.”

“But, but —” Mr. Wallace sputtered. There followed some to-and-fro, with Mr. Wallace eventually asking about the “tradition in this country” of “the peaceful transition of power” in which “the loser concedes to the winner.” He noted that he was not saying that Mr. Trump would necessarily be the loser but wanted to know, “Are you saying you’re not prepared now to commit to that principle?”

“What I’m saying is that I will tell you at the time,” Mr. Trump said.

Secretary Clinton called the answer “horrifying.” It left her on her back foot, though, and began the impression that it was Mrs. Clinton who lacked the stuff of presidents. In the event, the Democrats did not support a peaceful transition. On January 5 in the Oval Office, President Obama set in motion the plot to undercut the president-elect. By May 2017, Mrs. Clinton had formally declared herself part of the “resistance.”

Which brings us back to the Democrats trying to get Mr. Trump, yet again, to commit to a peaceful transition. And Mr. Trump, yet again, hanging back. He’s already sworn the presidential oath, which requires him to faithfully execute the office of president and to preserve, protect and defend the Constitution. He knows that it’s only the Democrats who have refused to accept the results of a recent presidential election. So it wouldn’t be surprising were he to resent the question.