USA final Presidential debate.

Donald Trump is ending his presidential bid the same way he began it: defiant and provocative, breaking campaign norms and ignoring experts’ advice — and becoming the nation’s first presidential nominee in memory to say he wouldn’t necessarily accept the election returns on Nov. 8.

Donald Trump at the conclusion of the third presidential debate in Las Vegas on Oct. 19, 2016.

At the third and final debate in Las Vegas on Wednesday, Trump ticked off a list of grievances: Voter fraud he claims could let millions of ineligible people cast ballots.

Charges of sexual misconduct against him that he said were “probably” generated by opponent Hillary Clinton’s campaign.

A news media he labeled as corrupt. And the stunning suggestion that he just might not concede the election if he lost.

Clinton called that response “horrifying.”

With charge and countercharge — each accusing the other of being “unfit” for the White House — it was the most brutal presidential debate of the television age. The candidates did not shake hands before the debate began or when it ended.

Voters looking for an uplifting conclusion to an unrelentingly negative campaign didn’t get it here.

At this point, winning the White House would require the billionaire businessman and reality TV star to stage the most dramatic political comeback in modern American history.

Clinton’s national lead is the largest any candidate has held in decades three weeks before Election Day.

But his fierce new message — that journalists and the political establishment are working in collusion against him to rig the election — helps guarantee that his campaign will have a continuing impact even if Clinton prevails.

Hillary Clinton speaks during the third presidential debate in Las Vegas on Oct. 19, 2016.

In some ways, Trump is like a bull trying to make sure as many teacups as possible are shattered before he leaves the china shop.

Indeed, he didn’t seem to be making much of an effort during the debate to reach out to independents and other swing voters by adopting a more presidential mien, or to reassure skeptical voters that he had the temperament for the job he seeks. “Such a nasty woman,” he muttered while Clinton was speaking.

Citing investigations into her use of a private email server when she was secretary of State, he called her a “liar” who “shouldn’t be allowed” to run for president.

He flatly denied allegations of aggressive sexual misconduct from about a dozen women who have come forward since the second debate. “It was lies and it was fiction,” he said. “I think they want either fame or her campaign did it,” pointing at Clinton.

“Every time Donald doesn’t think things are going in his direction, he claims everything is rigged against him,” Clinton said. She said she was “appalled” by his unwillingness to promise to accept the election results: “He is denigrating; he’s talking down our democracy.”