U.S. House Speaker Nancy Pelosi said the House will proceed with bringing legislation to impeach President Donald Trump to the floor – if Vice President Mike Pence doesn't invoke the 25th amendment – in a letter late Sunday to colleagues.

"In protecting our Constitution and our Democracy, we will act with urgency, because this President represents an imminent threat to both," said Pelosi, adding that "the horror of the ongoing assault on our democracy perpetrated by this President is intensified and so is the immediate need for action."

The House is set to vote on Monday or Tuesday on a resolution urging Pence to invoke the 25th Amendment of the U.S. Constitution, which allows the removal of the president if he is unable to discharge his official duties, according to the letter. 

Pence is not expected to take the lead in forcing Trump out, although talk has been circulating about the 25th Amendment option for days in Washington.

Republicans condemn Trump

With impeachment planning intensifying, two Republican senators said they want Trump to resign immediately as efforts mount to prevent him from ever again holding elective office in the wake of deadly riots at the Capitol.

House Democrats were expected to introduce articles of impeachment on Monday. The strategy would be to condemn the president's actions swiftly but delay an impeachment trial in the Senate for 100 days. That would allow President-elect Joe Biden to focus on other priorities as soon as he is inaugurated on January 20.

Representative Jim Clyburn, the third-ranking House Democrat and a top Biden ally, laid out the ideas on Sunday as the country came to grips with the siege at the Capitol by Trump loyalists trying to overturn the election results.

"Let's give President-elect Biden the 100 days" at the start of his term to deal with the most urgent issues, said Clyburn. "Maybe we'll send the articles some time after that."

Republican Senator Pat Toomey of Pennsylvania on Sunday joined fellow Republican Senator Lisa Murkowski of Alaska in calling for Trump to "resign and go away as soon as possible," saying the president could face criminal liability after the deadly storming of the U.S. Capitol by his supporters.

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

Late Saturday, Pelosi convened a conference call with her leadership team and sent a letter to her colleagues reiterating that Trump must be held accountable. 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.

"It is absolutely essential that those who perpetrated the assault on our democracy be held accountable," Pelosi wrote. "There must be a recognition that this desecration was instigated by the President."

Senate Majority Leader Mitch McConnell has said an impeachment trial could not begin under the current calendar before inauguration day.

Potentially complicating Pelosi's decision about impeachment is what it would mean for Biden. While reiterating that he had long viewed Trump as unfit for office, the president-elect said on Friday what Congress does "is for them to decide," when talking about the impeachment.

Democrats weigh options

Another idea being considered by Democrats was to have a separate vote that would prevent Trump from ever holding office again. That could potentially only need a simple majority vote of 51 senators, unlike impeachment, in which two-thirds of the 100-member Senate must support a conviction.

Democratic Representative David Cicilline, a leader of the House effort to draft impeachment articles accusing Trump of inciting insurrection, said Sunday that his group had 200-plus co-sponsors.

The 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 U.S. president had been impeached twice.

A violent and largely white group of Trump supporters overpowered police, broke through security lines and windows and rampaged through the Capitol last Wednesday, forcing lawmakers to scatter as they were certifying Biden's victory over Trump in the Electoral College.

Meanwhile, Marriott International Inc, America's largest hotel company, and the Blue Cross Blue Shield Association said Sunday they will suspend donations to U.S. lawmakers who voted against certifying Biden's victory.

(With input from agencies)