Washington: Hillary Clinton and Donald Trump are in a virtual dead heat in their bitter race for the White House on the eve of their first head-to-head presidential debate, a new poll showed on Sunday.

The Washington Post-ABC News poll found that Clinton’s slim margin from last month has now vanished. Instead, the Democrat and her Republican rival tied at 41 per cent support among registered voters, with Libertarian Party nominee Gary Johnson at 7 per cent and Green Party nominee Jill Stein at 2 per cent.

In a two-way matchup, Clinton and Trump each got 46 per cent among registered voters.

While some other national surveys show Clinton with a lead, poll averages show a low, single-digit margin.

Gender, race and education were clear markers between the two candidates.

Men back Trump by 54 per cent to 55 per cent of women who said they support Clinton, the poll showed. Some 53 per cent of white voters back Trump, compared to 37 per cent for Clinton, while non-white voters back her 69 per cent to 19 per cent for her Republican rival.

Trump is ahead of Clinton by more than four to one among white men without college degrees, a gap that narrows for white women without college degrees and college-educated white men.

Among college educated white women, Clinton leads Trump in the poll by 57 per cent to 32 per cent.

The two candidates are still grappling with high negativity among voters.

Around 39 per cent of registered voters see Clinton favourably, compared to 57 per cent who have an unfavourable impression.

For Trump, 38 per cent had a positive impression and 57 per cent a negative one, a rating five points lower than it was before the two parties’ national conventions in July.

While voters find both candidates lacking in honesty overall, Clinton’s ratings were worse, with just 33 per cent of voters finding her honest and trustworthy and 66 per cent saying she is not.

Some 42 per cent of voters said Trump was honest and trustworthy, while 53 per cent said he was not.

Most voters [53 per cent] who do not see Trump as qualified to be president, while 58 per cent said he lacks the temperament to be an effective president and 55 per cent said he lacked the world knowledge required for the job.

Meanwhile, Trump threatened on Saturday to bring a former lover of Clinton’s husband, Bill, Clinton to the debate, one of a series of references he has made to the ex-president’s infidelities.

His gibe came after billionaire investor Mark Cuban, a vocal Trump critic who backs Clinton, agreed to sit at the front of the audience for the televised debate, which is expected to shatter audience records with up to 90 million Americans watching.

“If dopey Mark Cuban of failed Benefactor fame wants to sit in the front row, perhaps I will put Gennifer Flowers right alongside of him!” tweeted Trump, himself a billionaire real estate investor.

Further complicating matters, a Twitter account linked to Flowers’s official website said she would attend the highly anticipated showdown — and suggested she backed Trump.

“Hi Donald. You know I’m in your corner and will definitely be at the debate! ...” the tweet read, with a kiss emoji at the end.

Bill Clinton had an infamous affair with Flowers during his tenure as Arkansas governor that he later admitted under oath.