US House Speaker Nancy Pelosi says the House “will proceed” with bringing legislation to the floor to impeach President Donald Trump, calling him a threat to democracy after the deadly assault on the Capitol.

Ms Pelosi made the announcement in a letter to colleagues. She said the House will act with solemnity but also urgency with just days remaining before Mr Trump is to leave office on January 20.

“In protecting our Constitution and our Democracy, we will act with urgency, because this President represents an imminent threat to both,” she said.

“The horror of the ongoing assault on our democracy perpetrated by this President is intensified and so is the immediate need for action.”

Late Saturday, Pelosi had convened a conference call with her leadership team and sent a letter to colleagues reiterating that Mr Trump must be held accountable for fomenting the violence by his supporters at the Capitol on Wednesday.

She told her caucus, now scattered across the country on a two-week recess, to “be prepared to return to Washington this week” but did not say outright that there would be a vote on impeachment.

• Calls to impeach Trump
• The hypocrisy of the ‘patriots’ who stormed Capitol
• Capitol stormed by angry mob

House Democrats were expected to introduce articles of impeachment on Monday and vote as soon as Tuesday local time.

Jim Clyburn, the third-ranking House Democrat and a top Biden ally, laid out a plan on Sunday to condemn the President but delay an impeachment trial in the Senate for 100 days.

That would allow President-elect Joe Biden to focus on other priorities when he is inaugurated on January 20.

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Meanwhile, Republican Senator Pat Toomey of Pennsylvania joined Republican Senator Lisa Murkowski of Alaska on Sunday in calling for Mr Trump to “resign and go away as soon as possible.”

“I think the president has disqualified himself from ever, certainly, serving in office again,” Mr Toomey said. “I don’t think he is electable in any way.”

Senate Majority Leader Senator Mitch McConnell has said an impeachment trial could not begin under the current calendar before Inauguration Day on January 20.

While many Republicans have criticised Mr Trump, most feel that impeachment would be divisive in a time of unity.

Still, some Republicans might be supportive.

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Nebraska Senator Ben Sasse said he would take a look at any articles that the House sent over.

Illinois Rep Adam Kinzinger, a frequent Trump critic, said he would “vote the right way” if the matter were put in front of him.

The impeachment articles, if passed by the House, could then be transmitted to the Senate for a trial, with senators acting as jurors to acquit or convict Trump. If convicted, Trump would be removed from office and succeeded by the vice president. It would be the first time a US president had been impeached twice.

-AAP