Republican presidential nominee Donald Trump said on Thursday he would accept the result of the Nov. 8 election - "if I win" - fuelling 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 Rep. Patrick Murphy, who is trying to unseat Republican Sen. 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.
Sen. 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McCain has withdrawn his support for Trump.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?"