US President Donald Trump faced allegations that he paid little or no income tax for years before he came to power, as his cloudy financial past stoked controversy ahead of the first election debate.

The New York Times alleged the billionaire president paid just $750 federal income taxes in 2016, the year he won the White House, and no income taxes at all in 10 of the previous 15 years because he reported losing more money than he made.

Mr Trump, who immediately dismissed the accusations as "totally fake news," is readying to come face to face with his Democratic opponent Joe Biden at a live debate tomorrow.

The Republican leader has broken with presidential tradition by refusing to release his tax returns, fighting a long battle in the courts and triggering speculation about what they might contain.

"First of all, I paid a lot and I paid a lot of state income taxes too... It'll all be revealed," Mr Trump said as he shrugged off the Times story that cited tax data extending more than 20 years.

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The paper said the records "reveal the hollowness, but also the wizardry, behind the self-made-billionaire image" of the president.

Senior Democrat Chuck Schumer took to Twitter, telling everyone who "paid more in federal income tax than President Trump" to raise their hand.

At tomorrow's election debate, millions of Americans will watch as the two antagonists - who depict each other as existential threats to the country - debate live on television.

Mr Trump taunted Mr Biden with the fresh salvo on his mental acuity. "I will be strongly demanding a Drug Test of Sleepy Joe Biden prior to, or after, the Debate on Tuesday night," he tweeted, saying he would take one also.

"His Debate performances have been record setting UNEVEN, to put it mildly. Only drugs could have caused this discrepancy???"

When asked by reporters about the demand, Mr Biden laughed before declining to comment.

Both men are prone to blunders and gaffes when speaking - but the 74-year-old Donald Trump has repeatedly depicted the 77-year-old Joe Biden as senile.

On Saturday, Mr Biden said he expects "personal attacks and lies" from the president, likening Mr Trump to Nazi propaganda chief Joseph Goebbels.

The former vice president has until recently stayed close to his Delaware home due to the challenges of safely campaigning in person for the 3 November election during the pandemic.

Joe Biden

Mr Trump, meanwhile, has been flouting his own government's social distancing guidelines to criss-cross battleground states, speaking frequently at mass rallies where participants are often tightly packed with few masks in sight.

The debate also comes as both sides try to exploit Mr Trump's bid to install conservative Amy Coney Barrett in the late Justice Ruth Bader Ginsburg's seat on the Supreme Court.

Mr Trump sees his nomination of Ms Barrett - potentially tilting the court to the right for years - as a fundamental boost to his troubled campaign.

He told Fox & Friends the Senate will "easily" confirm Ms Barrett before the election, despite furious Democratic opposition.

But Mr Biden hit back, accusing Mr Trump of rushing Ms Barrett's nomination in order to launch a new assault on health care.

He again urged the Senate to delay the confirmation until after the election, noting that early voting had already begun.

"Never before in our nation's history has a Supreme Court justice been nominated and installed while a presidential election is already underway," he said.

Barring a huge surprise, Republican senators, who have 53 out of 100 votes in the upper house of Congress, are expected to confirm Ms Barrett.

The TV debate could be a wildcard, with Mr Trump needing to break through the 200,000 US coronavirus deaths, the long-lasting economic fallout and fatigue at the constant upheaval roiling his administration.

He sees himself as a tough guy and has huge confidence in his prowess on stage.

Yet unlike the fawning treatment he enjoys during his weekly call-ins to Fox News or the adoring atmosphere at rallies, he will find himself facing a determined rival painting him as "toxic" for America.

"When Joe Biden walks onto the debate stage, it will be the first moment in four years where an American has the opportunity to confront Donald Trump for what he's done," Steve Schmidt, a Republican strategist turned outspoken Trump opponent, said on MSNBC.

Frontrunner Joe Biden mainly needs to keep steady against a man who many call a master provocateur.

"There is virtually no doubt that Trump will try to bait him," David Barker, director of the Center for Congressional and Presidential Studies at American University, said.