House Speaker Nancy Pelosi said Sunday the House will proceed with legislation to impeach President Donald Trump, calling him a threat to democracy after the deadly assault on the Capitol – unless the vice president and Cabinet invoke constitutional authority to force him out.

"I want to call to your immediate attention the action to be taken tomorrow morning, when Majority Leader Hoyer will request Unanimous Consent to bring up the Raskin resolution," Pelosi wrote. "This resolution calls on the Vice President to convene and mobilize the Cabinet to activate the 25th Amendment to declare the President incapable of executing the duties of his office, after which the Vice President would immediately exercise powers as acting President."

"If we do not receive Unanimous Consent, this legislation is planned to be brought up on the Floor the following day.  We are calling on the Vice President to respond within 24 hours," she added.

Pelosi made the announcement in a letter to colleagues, framing it as an ultimatum to Vice President Mike Pence to invoke the powers of the 25th Amendment to remove Trump from office. If not, she said, the House would proceed with impeachment. Trump could become the only president to be impeached twice.

“In protecting our Constitution and our Democracy, we will act with urgency, because this President represents an imminent threat to both,” she said, and added: “The horror of the ongoing assault on our democracy perpetrated by this President is intensified and so is the immediate need for action.”

Pelosi’s plan seeks a vote on Monday on a resolution calling on Pence and Cabinet officials to invoke the 25th Amendment. Under rules when the full House is not convened, any objection would reject the resolution. Pelosi would then put the resolution before the full House on Tuesday. If it were to pass, Pence and the Cabinet would have 24 hours to act before the House would move toward impeachment.

With impeachment planning intensifying, two Republican senators said they want Trump to resign immediately as efforts mounted to prevent Trump 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 Jan. 20.

Rep. Jim Clyburn, the third-ranking House Democrat and a top Biden ally, laid out the ideas 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 he needs to get his agenda off and running,” Clyburn said.

Pressure was mounting for Trump to leave office even before his term ended amid alarming concerns of more unrest ahead of the inauguration. The president whipped up the mob that stormed the Capitol, sent lawmakers into hiding and left five dead.

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

“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.”

Murkowski, who has long voiced her exasperation with Trump’s conduct in office, told the Anchorage Daily News on Friday that Trump simply “needs to get out.” A third Republican, Sen. Roy Blunt, of Missouri, did not go that far, but on Sunday he warned Trump to be “very careful” in his final days in office.

Corporate America began to tie its reaction to the Capitol riots by tying them to campaign contributions.

Blue Cross Blue Shield Association’s CEO and President Kim Keck said it will not contribute to those lawmakers — all Republicans — who supported challenges to Biden’s Electoral College win. The group “will suspend contributions to those lawmakers who voted to undermine our democracy,” Kim said.

Citigroup did not single out lawmakers aligned with Trump’s effort to overturn the election, but said it would be pausing all federal political donations for the first three months of the year. Citi’s head of global government affairs, Candi Wolff, said in a Friday memo to employees, “We want you to be assured that we will not support candidates who do not respect the rule of law.”

House leaders, furious after the insurrection, appear determined to act against Trump despite the short timeline.

Outgoing Senate Majority Leader Sen. Mitch McConnell (R-KY) has said an impeachment trial could not begin under the current calendar before Inauguration Day, Jan. 20.

While many have criticized Trump, Republicans have said that impeachment would be divisive in a time of unity.

Sen. Marco Rubio (R-FL) said that instead of coming together, Democrats want to “talk about ridiculous things like ‘Let’s impeach a president’” with just days left in office.

Still, some Republicans might be supportive.

Nebraska Sen. 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 Democratic effort to stamp Trump’s presidential record — for the second time — with the indelible mark of impeachment had advanced rapidly since the riot.

Rep. David Cicilline (D-RI), 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.

Potentially complicating Pelosi’s decision about impeachment was what it meant for Biden and the beginning of his presidency. While reiterating that he had long viewed Trump as unfit for office, Biden on Friday sidestepped a question about impeachment, saying what Congress did “is for them to decide.”

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

The Associated Press contributed to this report.