As Joe Biden and President Donald Trump square off for the first time on Wednesday (Australian time) all eyes will be on something unexpected – Mr Biden’s temperament, US experts have said.

Before the first staged debate between Mr Trump and his Democrat challenger, each campaign is promising a stark contrast in policy, personality and preparation.

Mr Trump has decided to skip any formal preparation.

Mr Biden’s team believes the significance of the debate may be exaggerated, but the Democratic nominee has been aggressively preparing to take on the President.

The backdrop could not be more dramatic.

The US death toll of COVID-19 has passed 200,000 and a damning report showing the President paid just $US750 in tax the year he ran for office was just released by the New York Times.

“I will be strongly demanding a Drug Test of Sleepy Joe Biden prior to, or after, the Debate on Tuesday night. Naturally, I will agree to take one also,” Trump wrote on Twitter.

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

In response, Mr Biden’s deputy campaign manager Kate Bedingfield said: “if the President thinks his best case is made in urine, he can have it”.

Although the challenge of a drug test might seem left of centre, it’s a calculated strategy from Mr Trump’s team, said United States Studies Centre research associate Elliott Brennan.

Tweet from @realDonaldTrump

“They have been aggressively setting the bar very low for Joe Biden in the past,” he told The New Daily. 

“If you read Trump’s tweets, probably up until the last week, you would think every time Biden takes a public stage there would be drool hanging out of his mouth.”

Mr Trump and his team have gone so hard on the ‘Sleepy Joe’ attack they’ve shot themselves in the foot – all Mr Biden has to do is sound coherent and he’ll win the debate, Mr Brennan said.

“As long as he is coherent, he will clear the bar the Trump campaign has set,” he said.

“That’s why you see the call for Biden to take a drug test. They’re trying to correct their tactical error.”

Mr Biden will be trying to pin Mr Trump on big-ticket items, like his handling of the pandemic and his tax returns.

Mr Trump will try to rile his opponent by attacking his family and raising questions about his son, Hunter Biden, and unsubstantiated accusations that Hunter’s work at a Ukrainian energy firm influenced US foreign policy. 

“Trump’s team knows Biden is loyal to his family and that he will get aggressive defending Hunter, so that’s something Trump will try to provoke,” Mr Brennan said.

US President Donald Trump paid only $US750 in federal income taxes for 2016 and 2017.

“Sometimes when Biden gets defensive, he can overstep boundaries. He’s challenged Democrats at town halls to push-up contests for instance.

“The challenge is he can get carried away and there’s a fine line between righteous anger and irrational anger.”

Mr Trump and Mr Biden are scheduled to meet on the debate stage for the first time on Tuesday night at Case Western Reserve University and the Cleveland Clinic in Cleveland, Ohio.

The 90-minute event moderated by Fox News host Chris Wallace is the first of three scheduled debates.

Mr Biden’s campaign has been holding mock debate sessions featuring Bob Bauer, a senior Biden adviser and former White House general counsel, playing the role of Trump, according to a person with direct knowledge of the preparations who spoke on condition of anonymity to discuss internal strategy.

“I’m sure the President will throw everything he can at (Biden),” said Jay Carney, a former aide to Mr Biden and President Barack Obama.

“My guess is that they’re preparing for that – bombarding him with insults and weird digressions.”

Mr Biden is currently the favourite, with poll leads in the national average.

For some, the debates represent the most important moments in the 2020 campaign’s closing days, a rare opportunity for millions of voters to compare the candidates’ policies and personalities side by side on prime-time television.

Mr Trump has been trailing Mr Biden in the polls for the entire year, a reality that gives the President an urgent incentive to change the direction of the contest on national television if he can.

Others, including those close to Mr Biden’s campaign, do not expect the debates to fundamentally change the race no matter what happens, given voters’ daily struggles with the pandemic and the economy.

But it will do little to change the outcome in November, Mr Brennan said.

“Presidential debates aren’t as influential as they seem,” he said.

“So many Americans have made up their minds. There’s a very slim margin of undecided voters and, on top of that, many have voted already.

“In terms of changing the result, this won’t be the pinnacle moment that advertisers and TV networks like us to believe.”

–with AAP