Pushing her confrontation with the coronavirus-sticken Donald Trump to a near crisis level, House Speaker Nancy Pelosi has announced that she will introduce legislation to create a panel to assess his competence to continue as the US president, an early step in the constitutional removal process.

Trump is in "an altered state", she asserted on Thursday, 26 days before the November 3 election.

"I don't know how to answer for that behaviour," the Speaker told Bloomberg News citing his decision to call off negotiations on the Covid-19 relief package and later indicating he wanted a partial deal.

"There are those who say when you are on steroids or have Covid19, there may be some impairment of judgment," said Pelosi, a Democrat who speaker is second in line to succeed the President.

Invoking the constitutional provision for removing a President from office, the Speaker and fellow-Democrat lawmaker Jamie Raskin said that they will introduce the bill on Friday for setting up a Commission on Presidential Capacity to Discharge the Powers and Duties that could recommend his removal from office.

The move is expected to be largely symbolic as the Republican-controlled Senate is unlikely to go along with such a measure passed by the Democrat-run House of Representatives in an attempt at a constitutional coup to remove Trump.

Trump shot back in a tweet, saying: "Crazy Nancy is the one who should be under observation. They don't call her crazy for nothing."

Later when asked about Pelosi's assertion about Trump being in "an altered state", White House Strategic Communications Director Alyssa Farah told reporters: "Absolutely not. The President's strong. He's working. He's never stopped."

Trump's personal doctor issued a memorandum on Thursday evening saying that he was stable and may be able to return to public engagements on Saturday.

Meanwhile in a joint statement, Raskin, a former constitutional law professor, and Pelosi said that the commision was "to help ensure effective and uninterrupted leadership in the highest office in the Executive Branch of government" under the Constitution's 25th Amendment.

That Amendment was passed in 1967 in reaction to the 1963 assassination of President John F. Kennedy to ensure that there was a definitive process for presidential succession.

Under the 25th Amendment, the Vice President becomes Acting President when the Vice President and a majority of either the cabinet or a body set up Congress send a declaration to the Speaker and President pro-tempore of the Senate that the President is unable to discharge the powers and duties of his office.

If the President challenges the declaration, a two-thirds majority of both chambers of the Congress will have to vote to remove him.

Some of its provisions were last followed in 1974 in setting up the succession when President Richard Nixon and his Vice President Spiro Agnew voluntarily resigned, although it was voluntary and did not need Congressional involvement.

An unnamed senior White House employee who wrote an op-ed for The New York Times in 2018 and later published a book, "A Warning", detailing what the writer claimed was the inner turmoil there, asserted that there were "early whispers" among some cabinet members about invoking the 25th Amendment against Trump.

Farah said that Trump, meanwhile, was back at work in the Oval Office on Thursday after having been mostly in the residential part of the White House since returning on Monday from the military hospital where he had been admitted last week for treatment.

She said that Trump had made some calls, including to Congress, and that he was committed to "skinny piecemeal" Covid-19 relief measures like helping airlines, aid for small businesses to keep employees and personal cheques to help individuals.

But he was against the $2.2 trillion package that Pelosi wants, she said.

Trump wants the $1,200 relief cheques to individuals go out before the election as it or an accompanying communication will carry his signature becoming an indirect appeal for support.

Pelosi will not allow it unless he agrees to a complete package that will include aid to states and local govenments, many controlled by her party that are in dire need.

Farah said that there would be a second debate, which would be in person, after Trump refused to attend the one scheduled for next Thursday that was to have been held virtually because of his Covid-19 infection.

Another debate was scheduled for October 22.

Rush Limbaugh, a right-wing radio talk show host said that Trump would hold a "virtual rally" on his programme on Friday.

Trump's doctor, Sean Conley, cleared Trump to campaign in person from Saturday.

"Saturday will be day 10 since Thursday's (October 2) diagnosis, and based on the trajectory of advanced diagnostics the (medical) team has been conducting, I fully anticipate the president's safe return to public engagements at that time."

That bill of health will be controversial because people are required in many states to quarantine for 14 days after merely travelling to a Covid-19 hot spot or being exposed to an infected person.

Trump is trailing his Democratic Party challenger Joe Biden by 9.7 per cent, according to the aggregation of polls by RealClearPolitics.

On Thursday, both parties focused on Arizona, a key swing state..