Donald Trump is insisting voter fraud does, indeed, pose a significant threat to the integrity of the US electoral system, claiming the election could be rigged "at the polling booths" despite a lack of evidence and experts' insistence to the contrary.

"Maybe just doesn't know how to win", said Trump in a taped interview with ABC before a campaign event in Green Bay, Wisconsin, Ryan's home state, on Monday.

Will the candidate echo his conspiracy claims in Vegas?

"Trump's participation ensures there shouldn't be many tiresome moments on Wednesday night", said Aaron Kall, director of debate at the University of MI.

Rep. Renee Ellmers (R-NC), a surrogate for Donald Trump, told CNN host Chris Cuomo on Tuesday that it was "not fair" to make her defend the GOP nominee's claim that USA elections were being "rigged" for Democratic nominee Hillary Clinton.

With Trump expected to attack Clinton, and vice versa, this confrontation figures to be a contrast to third debates of the past. There are a lot of times when things don't go our way or my way.

Since the second debate on October 9 - one in which Trump denied ever grabbing women in the manner discussed in a 2005 recording - more than a dozen women came forward to accuse the NY businessman of unwanted sexual advances.

That also holds true for any effort by hackers to influence this year's presidential election. He has described accusers as liars and criticized their looks. Clinton planned to spend Tuesday in NY preparing for the debate in Las Vegas. But they all support Trump's candidacy, as do the vast majority of Republican politicians. Thirty-nine percent of men also support Clinton.

If Trump loses, there will be a blame game as two camps float competing narratives that explain his loss: one in which he's a loser, and the other in which he's a martyr. "I mean, who can really know?" And while Trump surrogate Rudy Giuliani has said "dead people generally vote for Democrats rather than Republicans", he has not proffered any evidence to back up that statement.

The debate, moderated by Fox News' Chris Wallace, could mark the Republican nominee's last big chance to shift the momentum in the final three weeks of the race. Among the scheduled topics: debt and entitlements, immigration, economy, Supreme Court, "foreign hot spots" and "fitness to be president".

Despite the daily drip of revelations from her campaign chairman's hacked emails and persistent questions about her personal email use as secretary of state, Clinton has weathered those storms - in part by hammering her opponent over his temperament and personal past, including now-multiple allegations of sexual harassment and assault.

Trump seemed to be saying that the campaign of Democratic nominee Hillary Clinton had highlighted her modeling pictures.

"Trump still faced some venue and audience constraints during the St. Louis debate", Kall said. "They started. They started from the -from the beginning of the campaign putting my -my picture from modeling days", Trump said. At this point in 2012, showed Mitt Romney leading Barack Obama by less than half a point.