Donald Trump has mocked his much-derided comment at the US presidential debate that he might not accept the result of the election.
Mr Trump kicked off a rally in Delaware, Ohio, by saying that he "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".
But he added: "If I win."
Mr Trump is continuing to raise concerns about the integrity of the election, despite a lack of evidence of widespread voter fraud in the country.
Mr Trump said he will not commit to honouring the result of November's election because he wants to reserve his right to file a legal challenge "in the case of a questionable result".
He said "of course I would accept a clear election result" but stressed that he was not sure he would receive one, citing misleading statistics on voter fraud.
Mr Trump alleged without evidence that Hillary Clinton's campaign was trying to "rig" the election.
He made the unprecedented assertion during the final presidential debate on Wednesday.
Asked if he would concede if he loses the election, Mr Trump said: "I will look at it at the time."
Republican Senator John McCain, who lost the 2008 presidential election to Barack Obama, said he had conceded defeat "without reluctance" even though he did not like the outcome.
Mr McCain said he did not know who would win this presidential election. But he said the loser has always congratulated the winner and called him "my president".
Mr McCain said: "That's not just the Republican way or the Democratic way. It's the American way. This election must not be any different."
Mr McCain did not mention Mr Trump by name.