Donald Trump has challenged his rival Hillary Clinton to a drug test before their next debate, suggesting the Democrat was "pumped up" on performance-enhancing drugs in a stunning new twist to the brutal White House race.

Donald Trump | Source: Reuters | File Photo

The unsubstantiated attack from the Republican nominee came as a new poll out Sunday shows Clinton with a four-point lead over her Republican rival, suggesting that Trump still holds a solid core of voters despite a week of snowballing claims of sexual misconduct that have thrown his presidential campaign into chaos.

Trump has trampled all conventions in his treatment of his opponent, vowing if elected to jail her over her email practices as secretary of state -- and making "Lock Her Up" a rallying cry for his fired-up supporters.

His campaign has actively fueled right-wing conspiracy theories about Clinton's health, seizing on her bout of pneumonia last month to suggest she is concealing a major health problem, and is unfit for office.

In the latest attack, leveled without proof, Trump suggested she had taken drugs during their last debate, and called for testing ahead of their final duel Wednesday in Las Vegas.

At a rally Saturday in Portsmouth, New Hampshire, Trump said:

"At the beginning of her last debate, she was all pumped up at the beginning. At the end, it was like, 'Take me down,' she could barely reach her car,"

"Athletes, they make them take a drug test. I think we should take a drug test prior to the debate. Why don't we do that?" Trump said.

Hillary Clinton | Source: Reuters | File Photo

The Manhattan billionaire has spent the week claiming the media and a "global elite" are working against him. Ten women have now come forward to say they were the victim of unwanted advances by the real estate mogul in a recent slew of sexual harassment allegations against the 70 year old. But Trump denies the claims:

A confident Clinton has scaled back her campaign commitments, keeping a low profile as her rival battles the incendiary allegations, triggered by the release last week of a video of him bragging about groping women.

But the Clinton camp issued a swift response to Trump's latest comments on the election, accusing him of seeking to erode public faith in the vote.

"This election will have record turnout, because voters see through Donald Trump's shameful attempts to undermine an election weeks before it happens," her campaign manager Robby Mook said in a statement.

The virulence of Trump's attacks on the Clinton camp has raised concerns about whether the real estate mogul would even acknowledge a defeat, and how his legions of supporters would react should he lose.

President Barack Obama -- who along with First Lady Michelle Obama stepped up this week as a heavy-hitting surrogate for Clinton -- echoed those concerns at a rally Friday, warning democracy itself was at stake in next month's vote.

Barrack Obama with Hillary Clinton | Source: Reuters | File Photo

"In a democracy, you have a contest, but if you lose then you say congratulations and you move on," said the US leader, whose second term ends January 20.

Feature Image Source: Reuters