The third and last presidential debate in Las Vegas on Wednesday night seemed to be following a fairly familiar script of insults and charges sprinkled with policies, until something happened that we’ve never seen before: A defiant Donald Trump refused to say he would accept the results of the Nov. 8 election.

Debate moderator Chris Wallace of Fox News tried to underscore the gravity of the moment, evoking an essential tradition of the 240-year history of the nation — the peaceful transition of power — but the Republican nominee was adamant.

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“I will tell you at the time,” Trump said, then added almost flippantly, “I will keep you in suspense.”

And with that, he very likely cemented his loss in November. And, worse for the Republican Party whose banner he carries, the losses of many other candidates on his ticket.

This was a stunning moment.

Yes, he’s been saying for days lately that the election is being rigged against him, despite no reputable evidence that that’s the case. But he had been reprimanded for that by elected officials and election officials in his own party from across the nation. In the day before the debate, Trump campaign manager Kellyanne Conway and daughter Ivanka Trump both said he would accept whatever happens.

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More dramatically, running mate Mike Pence said just minutes before Trump walked onto the stage that both men would indeed accept those results.

And Trump made fools of them all. This was bad on several levels — in his embrace of conspiracy theories, in his trafficking in untruths, in his undermining of one of the bedrocks of American democracy, and in the fact that Trump once again put himself before the nation.

And where it gets really scary is that he has been complementing that warning with a request to supporters to go to polling places in other neighborhoods as a sort of citizens patrol. We have bipartisan poll watchers for that.

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For a candidate who by his demeanor early in the debate clearly showed he knew he needed to prove he has the temperament to be president, this was debilitating.

And to make matters worse for Trump, Hillary Clinton was ready to exploit his answer. She launched into a long recitation of Trump’s history of making “rigged” charges when things don’t go his way, including his failure to win some Emmys for “Celebrity Apprentice,” and she said that was his mindset.

Until then, Trump had been doing well.

He largely stayed in control, offered some good answers and made some effective attacks on Clinton, most notably on her 30 years of “bad” experience and conflicts of interest with the Clinton Foundation. But inevitably he started interrupting both Clinton and Wallace, went down some rabbit holes, and toward the end probably lost any women still on the fence when he responded to a Clinton aside about Trump trying to dodge paying taxes in the context of paying for entitlement programs by interrupting with a sneering, “Such a nasty woman.”

Clinton had a strong performance Wednesday night, as she did in the other two debates, possibly her best.

She had the lower bar — do no harm to her expanding lead in the polls, where Trump had to blunt her momentum. But she showed she has learned from their previous encounters, engaging him on some issues, ignoring him on others. She had her own moment, not that she needed one, in her emotional appeal to women everywhere who she said know what it’s like to be belittled and have their self-worth attacked.

But as strong as she was then, it’s Trump’s moment everyone will be talking about — from now until the election he now seems destined to lose.