WASHINGTON, Jan 7 (Reuters) - The top two Democrats in Congress on Thursday called for President Donald Trump's removal from office, one day after his supporters stormed the U.S. Capitol in a harrowing assault on American democracy.

With 13 days left in Trump's term, House of Representatives Speaker Nancy Pelosi and Senate Democratic Leader Chuck Schumer both said Vice President Mike Pence and the Cabinet should invoke the 25th Amendment of the U.S. Constitution to remove him from power before then.

Absent that, they said Congress should move quickly to expel him through the impeachment process.

"Yesterday the president of the United States incited an armed insurrection against America," Pelosi said at a news conference, adding that Trump posed an ongoing danger to the country.

Members of Trump's Cabinet and allies of the Republican president have discussed invoking the 25th Amendment, which allows them to remove a president who is unable to discharge the powers and duties of his office, a source familiar with the situation said. Another source said that was unlikely, however.

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Congress formally certified Democratic President-elect Joe Biden's election victory early on Thursday, hours after they were forced into hiding by hundreds of rioters who overwhelmed police and invaded the building. More than half of House Republicans and eight Republican senators voted to challenge the election results.

During the proceedings, Pelosi pulled Pence off the House floor to talk.

Meanwhile, Trump faced a staff exodus. One Cabinet official, Transportation Secretary Elaine Chao, who is married to top Senate Republican Mitch McConnell, said she would resign, citing the violence.

Other Trump officials, including envoy Mick Mulvaney, Trump's former chief of staff, also quit. More departures were expected.

Biden blamed the president for the attack but stopped short of calling for his ouster.

"He unleashed an all-out assault on the institutions of our democracy from the outset. And yesterday was but the culmination of that unrelenting attack," Biden said at a news conference.

Facebook said it would ban Trump posts until Biden's Jan. 20 inauguration. On Wednesday, Twitter suspended Trump's account for 12 hours.

CALLS FOR TRUMP TO LEAVE

Dozens of Democrats have called for Trump to be removed. At least two Republicans, Maryland Governor Larry Hogan and U.S. Representative Adam Kinzinger of Illinois, also said he should go, with Kinzinger saying in a video that Trump had become "unmoored" from reality.

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But absent action from the Cabinet, it was uncertain whether Congress - which is currently in recess - had time to start impeachment proceedings before Trump's term ends.

Several Democratic House members have already begun drafting articles of impeachment for Trump's role in encouraging Wednesday's lawlessness.

The Democratic-controlled House impeached Trump in December 2019 after he pressured Ukraine to investigate Biden, but the Republican-led Senate voted to acquit him in February 2020 on charges of abuse of power and obstructing Congress.

Trump pledged in an early-morning statement an "orderly transition" ahead of Biden's inauguration but has continued to claim falsely the election victory was stolen from him.

Trump has not condemned the extraordinary violence that unfolded after he encouraged supporters on Wednesday to march to the Capitol, despite pleas from senior members of his administration.

"I implore the President and all elected officials to strongly condemn the violence that took place yesterday," Acting Homeland Security Secretary Chad Wolf said.

Trump has increasingly isolated himself in the White House, relying on a small group of diehard loyalists and lashing out at Pence and others who dare to cross him, four sources said.

He has also told aides he is considering pardoning himself, the New York Times reported on Thursday. Constitutional scholars have said it is unclear whether the presidential pardon power can be used in that way.

Trump faces potential state legal actions when he leaves office, including a criminal probe in New York, that would not be covered by a federal pardon.

The assault on the Capitol was the culmination of months of divisive and escalating rhetoric by Trump and his allies around the Nov. 3 election, with the president repeatedly making false assertions that the vote was "rigged."

Rioters besieged the House chamber while lawmakers were inside. Security officers barricaded the chamber's door with furniture and drew their pistols before helping lawmakers and others escape.

New fencing was installed around the Capitol on Thursday ahead of Biden's inauguration.

Election officials of both parties and independent observers have said there was no significant fraud in the election. Biden received 7 million more votes than Trump in the national popular vote.

In a further setback to Trump, Democrats on Wednesday completed a sweep of the two U.S. Senate seats up for grabs in runoff elections in the state of Georgia, giving the party control of the chamber and boosting the prospects for Biden's legislative agenda.

(Additional reporting by Andrea Shalal, Matt Spetalnick, David Shepardson, Michael Martina, Lisa Lambert, Susan Heavey, Daphne Psaledakis, Susan Cornwell, Jan Wolfe and Trevor Hunnicutt Writing by Andy Sullivan and Joseph Ax Editing by Scott Malone, Frances Kerry, Will Dunham and Howard Goller)