House Speaker Nancy Pelosi has said President Donald Trump should immediately be removed from office or Congress may proceed to impeach him.
Ms Pelosi joined those calling on the Cabinet to invoke the 25th Amendment to force Mr Trump from office.
It came a day after a violent mob of Trump supporters stormed the Capitol, forcing the building into lockdown. Mr Trump called them "very special" people and said he loved them.
Ms Pelosi said at the Capitol: "The president of the United States incited an armed insurrection against America."
She warned he could do further harm to the country: "Any day can be a horror show for America."
Democrats and some Republicans want Mr Trump removed before his term ends on January 20 with Democrat Joe Biden's inauguration.
The 25th Amendment allows for the vice president and a majority of the Cabinet to declare the president unfit for office. The vice president then becomes acting president.
Republican congressman Adam Kinzinger and top Democrat Chuck Schumer have also called for the 25th Amendment to be invoked.
In another blow to Mr Trump, Transportation Secretary Elaine Chao said she is resigning, becoming the highest ranking member of the administration to quit in protest after the insurrection at the Capitol.
Ms Chao, who is married to Senate Republican leader Mitch McConnell, said the violent attack on the Capitol "has deeply troubled me in a way that I simply cannot set aside".
Mr Trump only acknowledged his election defeat and promised an "orderly transition" after a day of chaos and destruction on Capitol Hill as a mob of his supporters stormed the Capitol and unleashed unprecedented scenes of mayhem in hopes of halting the peaceful transition of power.
Members of Congress were forced into hiding, offices were ransacked, and the formal congressional count of Electoral College votes was halted for more than six hours.
"Even though I totally disagree with the outcome of the election, and the facts bear me out, nevertheless there will be an orderly transition on January 20," Mr Trump said in a statement posted to Twitter by his social media director.
Mr Trump's account had been locked by the company for posting messages that appeared to justify the assault on the seat of the nation's democracy.
Mr Trump added: "While this represents the end of the greatest first term in presidential history, it's only the beginning of our fight to Make America Great Again!"
However, President-elect Joe Biden called the mob that descended on the US Capitol "domestic terrorists" and laid the blame for the violence squarely at Mr Trump's feet.
During remarks in Wilmington, Delaware, on Thursday, Mr Biden said people should not call the hundreds of Trump supporters who broke into the Capitol protesters.
He said they were "a riotous mob — insurrectionists, domestic terrorists". He said Mr Trump is guilty of "trying to use a mob to silence the voices of nearly 160 million Americans" who voted in November.
He said the president has "made his contempt for our democracy, our constitution, the rule of law, clear in everything he has done" and unleashed an "all-out attack" on the country's democratic institutions that ultimately led to the violence on Wednesday.
The president has spent the past two months refusing to concede and making baseless allegations of widespread voter fraud, even though his own Justice Department, federal courts, including the Supreme Court, and state governments have said repeatedly the vote was carried out freely and fairly.
Mr Trump's refusal to accept reality and his incendiary rhetoric reached a breaking point on Wednesday when loyalists violently occupied the Capitol in one of the most jarring scenes ever to unfold in the nation's capital. Authorities said four people died during the violence, including one woman who was shot by an officer outside the House chamber.
Mr Trump had encouraged his supporters to march on the Capitol to protest politicians' actions, and he later appeared to excuse the violent occupation by the mob.
"These are the things and events that happen when a sacred landslide election victory is so unceremoniously & viciously stripped away from great patriots who have been badly & unfairly treated for so long," Mr Trump wrote in a message that was later deleted by Twitter.
He added: "Go home with love & in peace. Remember this day forever!"
Mr Trump spent much of Wednesday afternoon watching the insurrection on television from his private dining room off the Oval Office.
But aside from sparing appeals for calm issued at the insistence of his staff, he was largely disengaged.
Instead, a White House official said, most of Mr Trump's attention was consumed by his ire at Mr Pence, who defied Trump's demands by acknowledging he did not have the power to unilaterally reject the electoral votes that determine the next president.
Mr Trump only reluctantly issued the tweets and taped a video encouraging an end to the violence.
The posts came at the insistence of staff and amid mounting criticism from Republican politicians begging him to condemn the violence and tell his supporters to stand down, according to the official.
It's with a heavy heart I am calling for the sake of our Democracy that the 25th Amendment be invoked. My statement: pic.twitter.com/yVyQrYcjuD
— Adam Kinzinger (@RepKinzinger) January 7, 2021
On Thursday Mr Kinzinger, describing the president as unfit and unwell, called for the 25th Amendment to be used.
Mr Kinzinger said: "It's with a heavy heart I am calling for the sake of our Democracy that the 25th Amendment be invoked."
What happened at the U.S. Capitol yesterday was an insurrection against the United States, incited by President Trump.
This president must not hold office one day longer.
— Chuck Schumer (@SenSchumer) January 7, 2021
The quickest and most effective way—it can be done today—to remove this president from office would be for the Vice President to immediately invoke the 25th amendment.
If the Vice President and the Cabinet refuse to stand up, Congress must reconvene to impeach President Trump.
— Chuck Schumer (@SenSchumer) January 7, 2021
His call was echoed by Democratic Senate leader Chuck Schumer who said, if the Cabinet failed to act, Congress then should.
Mr Schumer tweeted: "The quickest and most effective way—it can be done today—to remove this president from office would be for the Vice President to immediately invoke the 25th amendment.
"If the Vice President and the Cabinet refuse to stand up, Congress must reconvene to impeach President Trump."
Even as authorities struggled to take control of Capitol Hill after protesters overwhelmed police, Mr Trump continued to level baseless allegations of mass voter fraud and praised his loyalists as "very special".
"I know your pain. I know your hurt.
"But you have to go home now," he said in a video posted more than 90 minutes after politicians were evacuated from the House and Senate chambers.
"We can't play into the hands of these people. We have to have peace.
"So go home. We love you. You're very special."
The violence, coupled with the president's tepid response, alarmed many in the White House and appeared to push Republicans allies to the breaking point after years of allegiance to Mr Trump.
A number of White House aides were discussing a potential mass resignation just two weeks before Mr Trump's term ends.
It has been an honor to serve the country in the @WhiteHouse . I am very proud to have been a part of @FLOTUS@MELANIATRUMP mission to help children everywhere, & proud of the many accomplishments of this Administration. Signing off now – you can find me at @OMGrisham ❤️🇺🇸
— Stephanie Grisham (@StephGrisham45) January 7, 2021
Stephanie Grisham, first lady Melania Trump's chief of staff and a former White House press secretary, submitted her resignation.
Deputy national security adviser Matt Pottinger, White House social secretary Rickie Niceta and deputy press secretary Sarah Matthews also resigned, according to officials.
Mick Mulvaney, Trump's former chief of staff-turned-special envoy to Northern Ireland told CNBC Thursday that he had called Secretary of State Mike Pompeo "to let him know I was resigning. ... I can't do it. I can't stay."