Donald Trump now says he will accept the election results - if he wins.
It's a different line to the one he took at the final presidential debate between Mr Trump and Hillary Clinton, when he refused to say he would accept the result of the election.
Speaking to supporters in Delaware, Ohio - a swing state which could go either way - Mr Trump announced he had "a major announcement" to make.
"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."
Mr Trump told the crowd if he does lose, he would only accept it if it was a "clear" result, but reserves his "right to contest or file a legal challenge in the case of a questionable result".
Donald Trump and Hillary Clinton at the third debate (Reuters)
During the debate on Thursday (NZ time) Mr Trump had been non-committal.
"What I'm saying is that I will tell you at the time," he told moderator Chris Wallace. "I'll keep you in suspense, okay?"
Ms Clinton said it was a "horrifying" thing to say.
"We are a country based on laws," she told media following the debate in Las Vegas.
"We've had hot, contested elections going back to the very beginning, but one of our hallmarks has always been that we accept the outcomes of our elections."
US news outlets pounced on Mr Trump's comments. The Associated Press said he was "threatening to upend a basic pillar of American democracy", CNN called it an "extraordinary departure from principles that have underpinned American democracy for more than two centuries" and the Washington Post headlined it an "attack on election integrity".
Mr Trump has repeatedly claimed the election is rigged.