By Patricia Zengerle and Emily Stephenson

WASHINGTON/DELAWARE, Ohio (Reuters) - U.S. Republican presidential nominee Donald Trump said on Thursday he would accept the result of the Nov. 8 election - "if I win" - fueling Republican concerns his stance would make it harder for his party to maintain control of Congress.

His refusal to commit to accepting the election outcome was the standout remark of the third and final 2016 presidential debate between Trump and Democratic rival Hillary Clinton on Wednesday night. It ratcheted up Trump's allegations the election was being rigged against him, and became the latest flashpoint in an unusually acrimonious race three weeks before voters go to the polls.

Clinton called the comment "horrifying."

President Barack Obama blasted Trump on Thursday at a rally in Miami Gardens, Florida, for Clinton and U.S. Representative Patrick Murphy, who is trying to unseat Republican Senator Marco Rubio, a Trump supporter.

"That is dangerous. Because when you try to sow the seeds of doubt in people's minds about the legitimacy of the elections, that undermines our democracy. Then you're doing the work of our adversaries for them," Obama said.

Trump modified his comment at a rally in Ohio on Thursday, but did not back off.

"I would like to promise and pledge to all of my voters and supporters and to all of the people of the United States, that I will totally accept the results of this great and historic presidential election - if I win," he said.

He added he would accept "a clear election result," but reserved the right to contest or file a legal challenge in the case of a questionable result.

With Trump trailing in opinion polls, the focus ahead of the Nov. 8 vote is shifting to whether Republicans can keep their narrow majority in the Senate or even their larger advantage in the House of Representatives.

Senator John McCain of Arizona, who lost the 2008 presidential election to Obama, said accepting the election result was "the American way."

"I didn’t like the outcome of the 2008 election. But I had a duty to concede, and I did so without reluctance," McCain, who has opened a poll lead in his Senate re-election race, said in a statement.

"A concession isn't just an exercise in graciousness. It is an act of respect for the will of the American people, a respect that is every American leader's first responsibility."

McCain has withdrawn his support for Trump.

Asked on Wednesday night if he would commit to a peaceful transition of power, Trump, a businessman-turned-politician, replied: "What I'm saying is that I will tell you at the time. I'll keep you in suspense. OK?"


Trump's statement, the most provocative in a debate that repeatedly descended into rancor, made headlines across the country and raised questions about his commitment to a peaceful transition of power, a cornerstone of American democracy.

Clinton accused Trump of being Russian President Vladimir Putin's puppet. Trump called Clinton a "nasty woman" and a criminal who should be barred from running. They did not exchange the customary handshake when the debate ended.

On Thursday night the two candidates appeared together at a formal charity dinner in New York and shook hands there after giving speeches intended to roast each other.

"Just before taking the dais, Hillary accidentally bumped into me, and she very civilly said, 'pardon me,'" Trump joked. "And I very politely replied, let me talk to you about that after I get into office." Trump has said Clinton should be imprisoned for her email practices as secretary of state.

Clinton spoke after Trump at the dinner, which she joked was remarkable in itself. "I didn't think he'd be ok with a peaceful transition of power," she said.

That show of humor was not the norm, however.

Democrats jumped to ask Republican candidates whether they agreed with Trump, who is making his first run for public office against Clinton, a former senator and first lady.

"Do you agree with Donald Trump to question the results of the election?" the Nevada Democratic Party asked in a release targeting Republican Joe Heck. Heck is in a tight race with Democrat Catherine Cortez Masto for retiring Democratic leader Harry Reid's Senate seat.

Trump's comments did not worry his supporters.

Marion Fields, 48, a registered Democrat who backs Trump, said he did not think concession would be an issue because Trump would win. Were he to lose, "After it's announced, you'd have to be a professional and concede."

A CNN/ORC snap poll said 52 percent of Americans thought Clinton won the debate, to 39 percent for Trump.

Trump donor and energy investor Dan Eberhart said Trump won. He disagreed with his rhetoric, but still backs the candidate.

"I think Hillary’s policies and track record are not what the country needs leading us forward for the next four years. And that backs me into supporting Trump," Eberhart said.

Trump campaign manager Kellyanne Conway sought on Thursday to defend Trump, saying in television interviews he was "putting people on notice" about voting irregularities.

Trump has not offered specific evidence to back up his vote-rigging claims, and numerous studies have shown that the U.S. election system, which is run by the states, is sound.

Trump has stepped up allegations that the election is being rigged against a backdrop of accusations that he had made improper sexual advances to women since a video emerged two weeks ago in which Trump boasted of such behavior.

A 10th woman came forward on Thursday at a news conference in Manhattan with attorney Gloria Allred, a Clinton supporter who specializes in representing women in cases of alleged assault.

Trump campaign spokeswoman Jessica Ditto called the news conference a "coordinated, publicity-seeking attack" by Allred.

First lady Michelle Obama, a powerful campaigner for Clinton, renewed her criticism of Trump without naming him during a campaign event in Arizona.

“Decent men do not demean women. And we shouldn’t tolerate this behavior from any man, let alone a man who wants to be the president,” she said.

(Additional reporting by Steve Holland in Las Vegas, Michelle Conlin, Luciana Lopez, Amanda Becker and David Ingram in New York, Roberta Rampton in Miami and Richard Cowan, Jeff Mason, Alana Wise, Doina Chiacu and Susan Heavey in Washington; Writing by Patricia Zengerle; Editing by Peter Cooney and Nick Macfie)