House Speaker Nancy Pelosi unveiled plans Thursday to probe Donald Trump's capacity to govern after he contracted Covid-19, as the US president unspooled a rant against critics and threw the debate schedule with Joe Biden into turmoil.

With just 26 days until the November 3 election, Washington's top Democrat took the extraordinary step of proposing a commission to probe Trump's fitness for the job -- and whether he needs removal under the Constitution's 25th Amendment.

But with tensions building over Trump's diagnosis and questions about his judgment, his doctor gave him the green light to resume public activities this weekend, opening the door for Trump's return to the campaign trail.

Joe Biden and Donald Trump
US President Donald Trump (right) accused the bipartisan debate commission of trying to "protect" his opponent Joe Biden. Photo: AFP / JIM WATSON

"Saturday will be day 10 since Thursday's diagnosis, and based on the trajectory of advanced diagnostics the team has been conducting, I fully anticipate the president's safe return to public engagement at that time," Trump's physician Sean Conley said in a statement.

Having been held back from campaigning, Trump raged on Fox Business television, insulting Biden's running mate Kamala Harris as a "monster," branding illegal immigrants "rapists," and urging indictments of Biden and former president Barack Obama.

And in remarks that caught Pelosi's attention, the 74-year-old Trump quipped that he beat Covid because "I am a perfect physical specimen and I'm extremely young."

Pelosi warned that Trump is suffering from a "disassociation from reality (that) would be funny if it weren't so deadly."

A Trump supporter
US President Donald Trump is failing to catch up with Democrat Joe Biden in the polls. Photo: AFP / GEORGE FREY

Senior House Democrat James Clyburn cautioned on CNN that Trump was exhibiting "very erratic behavior" that has drawn public concern.

As they questioned the president's claim to be rapidly recovering from Covid-19 and Pelosi announced her upcoming probe, Trump fired back on Twitter.

"Crazy Nancy is the one who should be under observation," he wrote. "They don't call her Crazy for nothing!"

Kamala Harris and Mike Pence
US Democratic vice presidential nominee Kamala Harris and US Vice President Mike Pence debated with a barrier between them. Photo: AFP / Robyn Beck

Trump's rejection of next week's debate because organizers decided to go virtual due to his bout with Covid-19 upended the calendar of debates -- usually a set piece series of three that candidates arrange well in advance.

After back and forth between Trump and Biden's campaign, it appeared likely that only two debates will take place in total, with the next being October 22 and the one scheduled for Miami on October 15 now scrapped.

With Biden surging in opinion polls and able to travel -- the veteran Democrat visited Arizona Thursday where he and Harris launched a campaign bus tour -- these are anxious times for Trump.

Nancy Pelosi
US Speaker of the House Nancy Pelosi intensified the debate about President Donald Trump when she announced a proposed commission to study the president's capacity to govern in the aftermath of his Covid-19 diagnosis leaves er her weekly press briefing on Capitol Hill in Washington, DC, on October 8, 2020. Photo: AFP / ANDREW CABALLERO-REYNOLDS

He is still recovering from his three-night hospital stint, while the White House itself has become a viral hotspot, with dozens of people close to Trump testing positive.

Trump's decision to boycott next week's debate, which would have been in town hall format with audience members asking questions, will mean missing a rare opportunity to try and best Biden in a direct televised confrontation.

Trump told Fox Business that the bipartisan debate commission's decision to make the debate a virtual affair was "not acceptable."

He accused organizers of trying to "protect" Biden after their angry first debate in Cleveland on September 29. Campaign manager Bill Stepien called organizers "pathetic" and announced that a rally would be held instead.

At the Biden campaign, spokeswoman Kate Bedingfield accused Trump of not wanting "to face questions from the voters about his failures on Covid and the economy."

Both camps agreed the next and probably final debate, on October 22 in Nashville, should be done in the town hall style.

Trump's campaign called for a third debate taking place five days before the election but Biden's side rejected this, saying "Trump's erratic behavior does not allow him to rewrite the calendar."

While Trump said he beat Biden "easily" in their first debate, opinion polls showed the president in a lopsided loss.

Biden is currently forecast to beat Trump in several vital swing states, even threatening him in Republican strongholds like Texas.

And Trump's personal fight with Covid-19 has thrown the spotlight back on an issue where polls find most voters see him as having failed.

The pandemic, which has claimed 212,000 American lives, has made it almost impossible for Trump to shift the campaign narrative back onto what he sees as more favorable territory: the economy, which was doing strongly before coronavirus hit early this year.

On Wednesday Harris debated with Vice President Mike Pence and spent much of her time hammering Trump for his pandemic response, calling it "the greatest failure of any presidential administration in the history of our country."

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