Donald Trump has claimed the US presidential contest is rigged against him, vowed to jail his Democratic rival Hillary Clinton if he is elected and also suggested she was on drugs during their last debate.
Mr Trump accused the media and the Clinton campaign of conspiring against him to undermine a free and fair election, claiming: "The election is being rigged by corrupt media pushing completely false allegations and outright lies in an effort to elect her (Mrs Clinton) president."
Mr Trump was referring to several women who have come forward in recent days to say that the Republican nominee had groped or sexually assaulted them. He has denied the claims, calling the women liars.
Mr Trump also took to Twitter to warn: "100% fabricated and made-up charges, pushed strongly by the media and the Clinton Campaign, may poison the minds of the American Voter. FIX!"
He added: "Hillary Clinton should have been prosecuted and should be in jail.
"Instead she is running for president in what looks like a rigged election."
In a country with a history of peaceful political transition, his challenge to the US election's legitimacy was a striking rupture of faith in American democracy.
Mr Trump has repeatedly claimed without offering evidence that election fraud is a serious problem and encouraged his largely white supporters to "go and watch" polling places in certain areas to make sure things are "on the up and up".
A prominent Trump supporter who spoke at the Republican convention last summer, Sheriff David Clarke Jr of Wisconsin's Milwaukee County, tweeted: "It's incredible that our institutions of gov, WH, Congress, DOJ, and big media are corrupt & all we do is bitch. Pitchforks and torches time."
Sheriff Clarke, an elected Democrat, illustrated his tweet with a photo showing angry people with clubs and torches.
House Speaker Paul Ryan, whose decision not to campaign for Mr Trump angered the party nominee, made clear he does not share the candidate's concern over the election's legitimacy.
A spokeswoman for Mr Ryan said: "Our democracy relies on confidence in election results, and the speaker is fully confident the states will carry out this election with integrity."
It is not the first time Mr Trump has raised the idea the election is unfairly tilted against him, but it has become a resurgent theme for the New York billionaire and many of his supporters in the past several days as he has slipped in the polls amid allegations of sexual misconduct.
Campaign money is also tight, at least in comparison with his rival's resources, according to information which pre-dates the release of a 2005 video that showed him bragging about imposing himself on women.
Mr Trump began this month with 75 million dollars (£61 million) in his campaign and joint party accounts. That is exactly half of what the Clinton team said it has on hand - a worrisome financial disadvantage for the Republican side.
There was trouble in Ohio, too, where Mr Trump severed ties with the state's Republican Party chairman, Matt Borges, who has been openly critical of the nominee.
That crack in unity comes in a critical battleground state, where Republican governor John Kasich has also failed to back Mr Trump.
Mr Trump's tribulations overshadowed the release of yet more emails hacked from accounts of Clinton campaign chairman John Podesta, laying bare aspects of her team's internal deliberations.
The latest batch showed the campaign worrying whether senator Elizabeth Warren might endorse Bernie Sanders, wrestling with how to respond to revelations about her private email use, and lining up materials to respond to fresh accusations from Juanita Broaddrick, who accused Bill Clinton of raping her decades ago.
He denied the rape accusation, which was never adjudicated by a criminal court.
Mr Trump also suggested Mrs Clinton had been on drugs during the last debate and challenged his rival to a drug test before their final televised tussle on Wednesday.
Instead of spending the weekend preparing, he claimed of his opponent: "I think she's actually getting pumped up, you want to know the truth.
"I think we should take a drug test prior to the debate, 'cause I don't know what's going on with her.
"But at the beginning of her last debate, she was all pumped up at the beginning. And at the end, it was like ... she could barely even reach her car."
Mr Trump offered no evidence to support his claim.