At least five Republicans have said they would join Democrats in voting for an article of impeachment – a formal charge – of inciting an insurrection just seven days before he is due to leave office and President-elect Joe Biden is sworn in on January 20th.
If the House approves it, Mr Trump would become the first president impeached twice.
A majority vote in the House to impeach would trigger a trial in the Republican-controlled Senate, although it was unclear whether such a trial would take place in time to expel Mr Trump from office.
As lawmakers debated the matter, National Guard troops and police were stationed around the Capitol to provide security.
House majority leader Steny Hoyer, the Number 2 Democrat, said Democrats intended to send the impeachment charge, once approved, to the Senate “as soon as possible,” and house speaker Nancy Pelosi named nine impeachment managers who would present the House’s case during a Senate trial.
Washington on high alert
The extraordinary swiftness with which Democrats were moving reflects the ongoing danger that Mr Trump poses to national security, according to top Democrats.
It also increases pressure on Mitch McConnell, the Republican Senate leader, to consider holding an immediate trial. Mr McConnell has said no trial could begin until the chamber returns from its recess on January 19th.
But Democratic minority leader Chuck Schumer, who is set to become the majority leader after two newly elected Democratic senators from Georgia are seated and Vice President-elect Kamala Harris is sworn in later this month, told reporters the Senate could be recalled to handle the matter if Mr McConnell agrees.
Washington is on high alert after the riot and with a week to go in Mr Trump’s term. Thousands of National Guard troops were to be on hand and some service members wearing fatigues, with weapons at hand, could be seen sleeping inside the Capitol building on Wednesday ahead of the session.
We are debating this historic measure at an actual crime scene
The House convened just after 9am (1400 GMT) in the same chamber where lawmakers hid under chairs last Wednesday as rioters clashed with police in the halls of the Capitol.
“We are debating this historic measure at an actual crime scene,” Democratic representative Jim McGovern said as the session opened. “This was a well-organised attack on our country that was incited by Donald Trump.”
Democrats moved forward on an impeachment vote after Vice President Mike Pence rejected an effort to persuade him to invoke the 25th Amendment of the US Constitution to remove Trump.
Trump’s Republican hold ebbs
As the House prepared for the impeachment vote, there were signs that Mr Trump’s hold on the Republican Party was beginning to ebb.
At least five House Republicans, including Liz Cheney, a member of her party’s leadership team, said they would vote for his second impeachment – a prospect no president before Mr Trump has faced.
“There has never been a greater betrayal by a President of the United States of his office and his oath to the Constitution,” Ms Cheney, the daughter of former Vice President Dick Cheney, said in a statement.
Mr Trump “summoned this mob, assembled the mob, and lit the flame of this attack” on the Capitol, she said.
Republicans Jaime Herrera Beutler, John Katko, Adam Kinzinger and Fred Upton also said they supported impeachment.
What I said was totally appropriate
In a break from standard procedure, Republican leaders in the House have refrained from urging their members to vote against impeaching Mr Trump, saying it was a matter of individual conscience.
The New York Times reported that Mr McConnell was said to be pleased about the impeachment push, another sign Mr Trump’s party is looking to move on from him after the attack on Congress.
Meanwhile, in his first public appearance since last Wednesday’s riot, Mr Trump showed no contrition on Tuesday for his speech shortly before the siege in which he called on his supporters to protest Mr Biden’s victory by marching on the Capitol.
“What I said was totally appropriate,” he told reporters.
At a meeting to set the rules for Wednesday’s impeachment vote, Democratic representative David Cicilline, who helped craft the impeachment measure, said it had the support of 217 lawmakers – enough to impeach Mr Trump.
A two-thirds majority of the Senate is needed to convict Mr Trump, meaning at least 17 Republicans in the 100-member chamber would have to vote for conviction.
Democrats could also use an impeachment trial to push through a vote blocking Mr Trump from running for office again. Only a simple Senate majority is needed to disqualify Trump from future office.