WASHINGTON -- US Republican presidential nominee Donald Trump said at a rally on Thursday he will totally accept election results "if I win."

"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," Trump told supporters in Delaware, Ohio, which is a key swing state in the the Nov. 8 election.

During the third and final debate with his Democratic rival Hillary Clinton on Wednesday night, the New York real estate billionaire refused to say if he will honor the election results, an unprecedented statement by a major party presidential nominee in the US history.

"I will look at it at the time," he said during the high-stake showdown in Las Vegas. "I'll keep you in suspense."

"Of course, I would accept a clear election result, but I would also reserve my right to contest or file a legal challenge in the case of a questionable result," Trump told the crowd on Thursday.

His comments ratcheted up his claims that has intensified in recent days that the election is being rigged against him.

The Republican nominee's refusal to commit to accepting the election outcome shocked many voters in the United States, where the concept that defeated candidates should concede to the winner and bow out graciously and quietly has been considered an inherent part of its political system.

Recent polls have shown Clinton has a steady double-digit lead over Trump after a leaked tape revealed Trump's lewd remarks about groping women earlier this month. The lead is more obvious among female voters, where Clinton enjoys a margin of 61 percent against 28 percent.