Pelosi has been raising questions about Trump's mental fitness since his COVID-19 diagnosis and demanding more transparency about his health.

The bill would set up a commission to assess the president's ability to lead the country and ensure a continuity of government.

It comes one year after Pelosi's House launched impeachment proceedings against Trump.

"This is not about President Donald Trump - he will face the judgment of the voters," Pelosi said at a press conference at the Capitol on Friday.

Just weeks before the November 3 election, with no hopes of the bill becoming law, the roll-out was quickly dismissed as a stunt by Trump's team and top allies.

"It's an absurd proposal," said White House press secretary Kayleigh McEnany on Fox.

"Absolutely absurd," said Senate Majority Leader McConnell during an appearance in Shepherdsville, Kentucky.

The president's opponents have discussed invoking the 25th Amendment for some time but are raising it now, so close to election day, as the campaigns are fast turning into a referendum on Trump's handling of the coronavirus pandemic.

The legislation that would create a commission as outlined under the 25th Amendment, which was passed by Congress and ratified in 1967 as a way to ensure a continuity of power in the aftermath of President John F Kennedy's assassination.

It says the vice president and a majority of principal officers of the executive departments "or of such other body as Congress" may by law provide a declaration to Congress that the president "is unable to discharge the powers and duties of his office".

At that point, the vice president would immediately assume the powers of acting president.

"Let Congress exert the power the constitution gave us," Pelosi said.

Congress is not in legislative session and so any serious consideration of the measure, let alone votes in the House or Senate, is unlikely.

Australian Associated Press