As his presidency imploded, Trump signalled a final, unrepentant display of division by announcing on Twitter that he will skip the inauguration of Joe Biden on January 20.

"To all of those who have asked, I will not be going," he tweeted.

Biden responded this was "a good thing," branding Trump an "embarrassment."

However, Biden showed how wary he is of the growing rush to impeach Trump -- and deepen the national political war -- over his incitement of crowds who stormed Congress on Wednesday.

"That is a judgment for the Congress to make," Biden said, adding that the "quickest" way to get Trump out was for him and Vice President-elect Kamala Harris to take over in 12 days.

"I am focused now on us taking control as president and vice president on the 20th and to get our agenda moving as quickly as we can."

Two days after Trump sent a mob of followers to march on Congress, his presidency is in freefall, with allies walking away and opponents sharpening their teeth.

House Speaker Nancy Pelosi warned that Democrats will launch impeachment proceedings unless Trump resigns or Vice President Mike Pence invokes the 25th Amendment, where the cabinet removes the president.

"If the President does not leave office imminently and willingly, the Congress will proceed with our action," Pelosi wrote.

In a jaw dropping moment, Pelosi revealed she had spoken Friday with the Chairman of the Joint Chiefs of Staff Mark Milley about "preventing an unstable president from initiating military hostilities or accessing the launch codes and ordering a nuclear strike."

"The situation of this unhinged President could not be more dangerous, and we must do everything that we can to protect the American people," Pelosi wrote.

Democrats in the House of Representatives, who already impeached Trump in a traumatic, partisan vote in 2019, said the unprecedented second impeachment of a president could be ready next week.

"We can act very quickly when we want to," Representative Katherine Clark told CNN.

Whether Republican leaders of the Senate would then agree to hold a lightning fast impeachment trial before the transition is another matter.

In the House, the senior Republican representative Kevin McCarthy said "impeaching the president with just 12 days left in his term will only divide our country more."

- Too little, too late -

Trump, whose actions on Wednesday capped his relentless efforts to overturn Biden's November 3 election win, finally conceded defeat on Thursday and appealed for calm.

"A new administration will be inaugurated on January 20. My focus now turns to ensuring a smooth, orderly and seamless transition of power," Trump said in a short video.

However, the evidently reluctant concession, in which Trump failed to congratulate Biden or directly admit defeat, was too little, too late to calm outrage over his role in the Capitol invasion.

Five people died in the mayhem, including one woman who was shot dead and a Capitol Police officer. Flags over the Capitol were lowered to half-mast on Friday.

Senator Ben Sasse, one Republican who says he will "definitely consider" impeachment, recommended that Trump at minimum step back and let his vice president run the show in the dying days.

"I think the less the president does over the next 12 days the better," he told NPR radio.

- Government exit -

Education Secretary Betsy DeVos became the second cabinet member to quit, after Transportation Secretary Elaine Chao, telling Trump in a letter that such "behavior was unconscionable for our country."

A string of lower level officials have also left. According to reports, the only reason the trickle hasn't turned into a flood is the decision by senior figures to try and maintain stability during the transition to Biden.

Trump, however, appears to have lost the grip he once exercised on both the Republican party and his own staff as he rampaged through four years of one of the most turbulent presidencies in US history.

Speaking to CNN, retired Marine Corps general John Kelly, who served as Trump's chief of staff for 18 months, said the cabinet should consider the 25th Amendment but believed the president had already been put into a box.

"He can give all the orders he wants but no one is going to break the law," Kelly said.

- Biden faces grim inauguration -

Biden, who won seven million votes more than Trump, as well as a decisive majority in the vital state-by-state Electoral College, will be sworn in on the Capitol Steps under huge security.

Between drastic Covid-19 crowd restrictions, the absence of Trump, and a new "unscalable" fence around the congressional complex, there will be little of the ordinary inauguration vibe.

And Biden will immediately face extraordinary challenges, starting with his core campaign promise that he can "heal" the nation.

But at the same time, the crisis has sparked such revulsion in Congress on both sides of the aisle that Biden may come into office with an unexpectedly bipartisan tailwind.

Biden said Friday that more Republicans now saw Trump for what he was after he "ripped the Band-Aid all the way off."

"I think it makes my job easier, quite frankly."