WASHINGTON - President Barack Obama issued a scathing rebuke Tuesday to Donald Trump on the eve of the final election debate, blasting him for "whining" about "rigged" elections and warning that such accusations are irresponsible.

Trump and Democratic rival Hillary Clinton are set for their final presidential debate Wednesday, with three weeks to go before Americans head to the polls on November 8. It is seen as a last chance for the Republican nominee, dogged by accusations of sexual misconduct and sinking poll numbers, to make his mark on millions of voters.

With the provocative billionaire's campaign reeling, Trump is likely to engage in more scorched-earth tactics if recent history is any guide.

But with Trump pressing the dangerous conspiracy theory that the US election is "rigged," Obama abandoned diplomatic decorum and skewered the mogul from the Rose Garden in front of visiting Italian Prime Minister Matteo Renzi.

"I have never seen in my lifetime, or in modern political history, any presidential candidate trying to discredit the elections and the election process before votes have even taken place. It's unprecedented," Obama told a joint press conference.

"That is both irresponsible - and, by the way, it doesn't really show the kind of leadership and toughness that you'd want out of a president. You start whining before the game's even over?

"If, whenever things are going badly for you and you lose, you start blaming somebody else? Then you don't have what it takes to be in this job," he added.

"I'd advise Mr Trump to stop whining, and go try to make his case to get votes."

Donald Trump, the Republican presidential candidate, has said that the controversy surrounding his Democratic rival Hillary Clinton’s emails, specifically the recent revelation that a State Department staffer asked to change the classification level of an email from her private server, is “worse than Watergate.”

Speaking to a rowdy crowd in Green Bay, Wisconsin, Monday night, Trump referred to Clinton’s dealings with the State Department and the Federal Bureau of Investigation (FBI) as “a criminal enterprise.”

“The Undersecretary of State Patrick Kennedy illegally pressured the FBI to un-classify emails from Hillary’s illegal server,” Trump said. “That’s a lot of illegality in that one statement.”

Meanwhile, one of the women who alleges Republican Presidential nominee Donald Trump harassed her, Mindy McGillivray, has said she intends to leave the country because she fears for her safety.  McGillivray came forward last week, stating she was outraged by Trump’s denial in the debate.

Since telling her story to the Press, however, McGillivray states she has come to fear for her and her family’s safety.

According to the Post, she’s been staying at a hotel, but when she returned to the house she shares with her daughter and stepfather, she was frightened by the attention her story has garnered.

“I look out the window and there are cars just driving around the house and looking, slowing down right at the house,’’ she said. “I don’t live in a gated community. This is dangerous. There could be people out there who want to hurt us.’’

Trump supporters have shown their willingness to follow Trump’s rhetoric with violence before.

Protesters at Trump rallies, often people of colour, have been punched, kicked, and choked. Riled up by Trump’s repeated statement that the election is “rigged” and is being stolen from him, supporters at a Trump rally in Cincinnati openly mused about assassinating Hillary Clinton and starting a “coup,” according to media reports.

On Thursday, two legally armed Trump supporters stood outside a Hillary Clinton campaign office for hours, stating they were protesting.

Trump denied the accusations and called them total fabrications.

Meanwhile, Melania Trump, who has been all but invisible as her husband confronted a campaign crisis, emerged on Monday to forcefully defend him and question the honesty of the women making the accusations.

Melania Trump echoed her husband’s complaint that he was the victim of a broad conspiracy between the news media and the Clinton campaign.

“I believe my husband, I believe my husband — it was all organised from the Opposition,” Ms. Trump said. “They can never check the background of these women. They don’t have any facts.”

But she stressed that she believed that Mr. Trump was simply being boastful and did not engage in such behaviour.