Donald Trump has said he will “totally accept” the result of the US presidential election “if I win”.
The Republican candidate has now admitted he will accept a clear result next month, but he would also reserve the right to file a legal challenge if he felt it was “questionable”.
Speaking at a rally in in Delaware, Ohio, he said: “I would like to promise and pledge to all of my voters and supports and to all of the people of the United States that I will totally accept the results of this great and historic presidential election.”
But he then added: “If I win.”
Mr Trump had previously refused to be drawn on the issue.
When pressed by the moderator of the third and final debate to confirm he will accept the outcome of the vote, he said: “I will look at it at the time.
“I will keep you in suspense.”
Hillary Clinton described his comment as “horrifying” and said it was extraordinary that the nominee of a major party would say such a thing.
The question related to Mr Trump’s recent insistent claims that the election is rigged.
Senator John McCain, who lost the 2008 presidential election to Barack Obama, said in a statement accepting the result is “the American way”.
“I didn’t like the outcome of the 2008 election,” he said.
“But I had a duty to concede, and I did so without reluctance.”
During the Las Vegas debate on Wednesday the candidates discussed issues such as Russia, abortion, immigration and foreign policy.
In the most drawn out confrontation of the evening, Mr Trump was challenged to admit to and condemn Russia’s suspected involvement in hacking emails and trying to interfere with the US elections.
The pair also clashed over immigration, with Mr Trump accusing Mrs Clinton of wanting open borders.
In a memorable line, he promised again to build a wall on the border with Mexico, because “we have some bad hombres here and we are going to get them out”.
President Obama hailed Mrs Clinton’s performance on Twitter.
He tweeted: “Outstanding 3 for 3 debate sweep for @HillaryClinton! Nobody has ever been more prepared to be @POTUS.”
Twitter said Mr Trump dominated conversations on the social media platform, claiming he was the subject of nearly 60% of the tweets sent about the pair.
It said the top tweeted moment was their discussion about the treatment of women, and the second most tweeted was Mr Trump’s refusal to say if he will accept the results should he lose.
There are now just 19 days until America goes to the polls.