Donald Trump sailed into another political tempest Thursday after threatening not to recognize the outcome of the US presidential election in a final debate with Hillary Clinton, the Democratic frontrunner.
With the November 8 elections just 19 days away, the face-off in Las Vegas was seen as the Republican nominee's last best chance to turn around a sinking White House bid.
But with millions watching on television, a defiant Trump turned what many thought began as his strongest debate performance yet into a gift to Clinton and another major headache for Republicans.
Asked point-blank whether he would accept the results of the elections no matter what, the 70-year-old reality television star said: "I'll tell you at the time. I'll keep you in suspense, OK?"
Clinton declared herself "appalled" by what she said was an attack on 240 years of US democracy.
And, quoting her former rival Bernie Sanders, she called Trump the "most dangerous person to run for president in the modern history of America."
Trump's shattering of political convention dominated US newspaper headlines and television coverage.
Republicans worried about the impact of Trump's remarks on Republicans in down-ballot races.
Reince Priebus, the chairman of the Republican National Committee, offered assurances on MSNBC. "Barring massive voter fraud, of course he is going to accept the results of the elections," he said.
But Republican Senator Jeff Flake said Trump was "beyond the pale" and onetime presidential candidate Senator Lindsey Graham said if Trump loses, it will be "because he failed as a candidate".
Democrats called on Republican leaders to repudiate "Trump's utter contempt for our democracy," as Nevada Senator Harry Reid put it.
"One of our hallmarks has always been we accept the outcome of our elections," Clinton told reporters as she flew home to White Plains, New York.
"So what he said tonight is part of his whole effort to blame somebody else for his campaign, and where he stands in this election."
Trump left Las Vegas immediately after the debate, heading to swing state Ohio for a day of campaigning.
He meets up with Clinton again at the end of the day in New York at the Al Smith Dinner, an annual charity event where the candidates traditionally engage in a "friendly roast."
But the animosity between them seems almost certain to get in the way.
They would not even shake hands at Wednesday night's debate, and at one point Trump interrupted Clinton to call her "such a nasty woman".
Clinton, who is vying to become the first woman president of the United States, told reporters she was "both relieved and very grateful" that the debates were now behind her.
She and husband Bill appeared arm in arm at an outdoor rally, basking in the cheers of some 5,000 supporters.
Polls show her leading by more than six points and making gains even in states like Arizona, Texas and Georgia that have long been in the Republican column.
"Hillary Clinton almost certainly will win the election, but the question is what is going to be the effect on Republican Senate, House and other candidates," said Robert Erikson, a political science professor at Columbia University.
On Thursday, Michelle Obama will be stumping for Clinton in Arizona and President Barack Obama will speak at a rally in Miami.
Obama earlier in the week told Trump to "stop whining" about a rigged election and go try to get people to vote for him.
But the New York billionaire plunged ahead anyway, paying no heed.
"The media is so dishonest and so corrupt and the pile-on is so amazing," Trump said, referring to reports citing women accusing him of sexual assault, which he said were "fiction" and drummed up by Team Clinton.
He alleged that millions of fake voters had been registered and that the 68-year-old Clinton should not even have been allowed to run because she mishandled classified State Department emails.
The former secretary of state scored an early hit against the Republican real estate mogul, alleging that Russian President Vladimir Putin was backing his run for office.
Trump argued that he might have better relations with Moscow than Clinton would, declaring: "Putin, from everything I see, has no respect for this person."
Clinton's response was sharp: "Well, that's because he would rather have a puppet as president of the United States."
Trump blustered back: "No puppet. You're the puppet."
Compared to previous brawls, the debate got off to a subdued but oddly substantive start.
Discussing their vision for the Supreme Court, Clinton insisted gay rights and women's rights must not be rolled back while Trump vowed to appoint anti-abortion justices who would also protect gun rights.
"If you go with what Hillary is saying, you can take the baby and rip the baby out of the womb of the mother just prior to the birth of the baby," he said.
"Using that kind of scare rhetoric is just terribly unfortunate," Clinton responded.