The US House of Representatives is expected to hold a vote to impeach President Donald Trump over his role in last week's storming of Congress.
Democrats accuse the president of encouraging his supporters to attack the Capitol building. Five people died.
Members of Mr Trump's Republican party say they will join Democrats to impeach him on Wednesday, formally charging the president with inciting insurrection.
President Trump has rejected any responsibility for the violence.
The riot last Wednesday happened after Mr Trump told supporters at a rally in Washington DC to "fight like hell" against the result of November's election.
Will Trump be impeached?
As Democrats hold a majority in the House, the vote is likely to pass. The case will then head for the Senate, where a trial will be held to determine the president's guilt.
A two-thirds majority would be needed there to convict Mr Trump, meaning at least 17 Republicans would have to vote for conviction. As many as 20 Senate Republicans are open to convicting the president, the New York Times reports.
The timeline of when a trial could be held is not known but it is unlikely it could be finished before Mr Trump leaves office on 20 January, when Joe Biden will be sworn in as president.
The Senate could also use an impeachment trial to block Mr Trump from ever running for office again. He has indicated he plans to campaign for president in 2024.
Wednesday's vote means that Mr Trump is likely to become the first US president ever to be impeached twice.
He became the third president to be impeached in December 2019 over charges of breaking the law by asking Ukraine to investigate Mr Biden in the election. The Senate cleared him.
Impeachment: The basics
What is impeachment? Impeachment is when a sitting president is charged with crimes. In this case, President Trump is accused of inciting insurrection by encouraging his supporters to storm the Capitol
Could Trump be removed from office? A simple majority of the House of Representatives is enough to impeach him - but to remove him from office, he then needs to be convicted of those crimes by the Senate, where a two-thirds majority is not guaranteed
So what does it mean? This is the second time Mr Trump will have been impeached, and even though a trial could begin after his term ends, a conviction could mean he is barred from holding public office again
What have Republicans said?
The third most senior Republican in the House, Liz Cheney, vowed to back impeachment, saying Mr Trump had "summoned this mob, assembled the mob, and lit the flame of this attack", referring to last week's riot.
"There has never been a greater betrayal by a president of the United States of his office and his oath to the Constitution," said the Wyoming representative, daughter of former Vice-President Dick Cheney.
At least four other Republican House members said they would also vote for impeachment.
House Republican leader Kevin McCarthy, a Trump ally who has said he opposes impeachment, decided not to ask rank-and-file members of the party to vote against the measure, US media reported.
According to the New York Times, Senate Republican leader Mitch McConnell told confidants he was pleased Democrats wanted to impeach the president because he believed it would help rid the Republican party of Mr Trump.
Trump's iron grip loosens
Analysis box by Anthony Zurcher, North America reporter
In the time it took Air Force One to fly Donald Trump back from the Texas border on Tuesday, the political ground crumbled beneath his feet. Mitch McConnell's signals that he is "pleased" with Democratic efforts suggest the political calculus is changing for Republican leaders in Congress.
A growing number believe the unrepentant president's actions last week threatened not just US democracy, but also their personal safety.
And even before the riot at the US Capitol, Mr Trump was increasingly viewed as a political liability with dwindling power. His scorched-earth challenge to the election results probably cost Republicans two Senate seats in Georgia, and there has been scant proof that Mr Trump boosts the party's electoral chances when his name is not on the ballot.
Mr McConnell, among others, may be mulling whether a clean break with Trump is better for their political futures, even if it means working with Democrats to do the job.
What did Trump say?
Speaking on Tuesday, Mr Trump showed no contrition for remarks he made to supporters before the violence, when he also repeated unsubstantiated claims of voter fraud.
"What I said was totally appropriate," Mr Trump told reporters. "I want no violence."
He also said: "This impeachment is causing tremendous anger... and it's really a terrible thing that they're doing," adding that the "real problem" was rhetoric used by Democrats during Black Lives Matter protests and violence last year.
In a separate development, YouTube said it had suspended Mr Trump's channel as it violated policies for inciting violence. The president's accounts on Twitter and Facebook have already been removed.
On Tuesday evening, House Republican Brian Fitzpatrick of Pennsylvania introduced a resolution to censure Mr Trump, a congressional rebuke less severe than impeachment.
The measure accuses Mr Trump of "trying to unlawfully overturn" the results of the election and of having "imperilled a coequal branch of Government".
Earlier, the House passed a resolution by 223-205 votes calling on Vice-President Mike Pence to help oust Mr Trump using the 25th Amendment, which would allow the cabinet to remove the president if he is deemed unable to discharge his duties.
But Mr Pence had already rejected the Democrats' resolution, saying in a letter to House Speaker Nancy Pelosi: "Under our Constitution, the 25th Amendment is not a means of punishment or usurpation."