WASHINGTON (AP) — 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.

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 Trump is to leave office on Jan. 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.”

Pelosi said that first the House will try to force Vice President Mike Pence and the Cabinet to oust Trump by invoking the 25th Amendment.

On Monday, House leaders will work to swiftly pass legislation to do that. If it is blocked by Republicans, which is almost certain, the House will convene for a full House vote Tuesday.

Pelosi explained that the resolution calls on Pence “to convene and mobilize the Cabinet to activate the 25th Amendment to declare the President incapable of executing the duties of his office.” Under the procedure, the vice president "would immediately exercise powers as acting President,” she wrote.

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.

Next, the House would move to consider the articles of impeachment, Pelosi said. The day for an impeachment vote was not set.

With impeachment planning intensifying, two Republican senators said they want Trump to resign immediately as efforts mount 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.

Late Saturday, Pelosi, D-Calif., 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 Sen. Mitch McConnell, R-Ky., has said an impeachment trial could not begin under the current calendar before Inauguration Day, Jan. 20.

Clyburn said that Pelosi "will make the determination as when is the best time” to send articles of impeachment to the Senate if and when they are passed by the House.

Another idea being considered 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.

The Senate was set to be split evenly at 50-50, but under Democratic control once Vice President-elect Kamala Harris and the two Democrats who won Georgia's Senate runoff elections last week are sworn in. Harris would be the Senate's tie-breaking vote.

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

Sen. Marco Rubio, R-Fla., 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-R.I, 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.

Toomey appeared on CNN's “State of the Union” and NBC's “Meet the Press.” Clyburn was on “Fox News Sunday” and CNN. Kinzinger was on ABC's “This Week,” Blunt was on CBS' “Face the Nation” and Rubio was on Fox News Channel's “Sunday Morning Futures.”

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Superville reported from Wilmington, Delaware. Associated Press writers Alexandra Jaffe, Lisa Mascaro and Zeke Miller contributed to this report.