Trump was repeatedly pressed by moderator Chris Wallace of Fox News to say whether he would accept the results of the election.
“I will tell you at the time. I will keep you in suspense, okay?” Trump finally said.
Fox News didn't beat around the bush, as its post-debate coverage began, in acknowledging this was the stand-out moment of the night -- and not in a good way for the GOP nominee.
"The number one most discussed issue on social media tonight was Donald Trump's unwillingness to accept the legitimacy of the results of the election," Megyn Kelly said, while repeating his remarks. "That had the online equivalent of a jaw drop among many Republicans even, as trouble that he didn't need to stir up for himself and to take away from what was otherwise a fairly solid debate performance."
In the same vein, other conservative pundits framed the remark as a devastating blemish on an otherwise above-average (for Trump) debate performance -- though at least a few were willing to point out its dangers to democracy as well.
"When you say that you’ll consider it, you’ll think about it. To me, that’s a showstopper. I don’t know how you get past that," said Juan Williams. "That’s not in our American tradition. That’s not the way we play politics."
"He’s the guy that really needed to pull something off tonight, and even if he did, and he may well have in the eyes of many people, I think he stepped on it by refusing to say he would accept the results," Hume said.
Chris Stirewalt ticked off his own list of strong moments for Trump, before turning to election answer.
"Unfortunately for him, he said the thing about the elections and that is going to blow up everything else."
Fox News figures were also quick to point out that Trump had put himself explicitly at odds with his own surrogates and advisers.
"I know they went over this in debate prep. And obviously Kellyanne Conway and others have out there publicly saying, 'No no no. We’re not challenging. We’re just talking about the unfair media and Trump gave the answer he wanted to give,'" Howard Kurtz said, adding that, before that moment, he thought it was Trump's best general election debate performance yet.
"But with that one answer — sometimes it takes one minute out of the 90 — Trump reset it. That’s going to be the big story," Kurtz said.
"He blew it up by a totally wrong answer on accepting the results," Charles Krauthammer said, calling it "a terrible mistake."
"This was not a gaffe where you say something off the cuff and it's what you think, but it's wrong. You know that he had been coached on this. You know that his vice president had said, 'Of course, we'll accept the result,' and his campaign manager and his daughter," Krauthammer said. "You know he's convinced this is something you wants to take a stand on. The calculation — this is political suicide."