The US House of Representatives is deciding whether 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.
Some in 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.
As the House continued its debate, Mr Trump responded to the latest reports of planned protests, urging calm.
"I urge that there must be NO violence, NO lawbreaking and NO vandalism of any kind," he said in statement released by the White House.
"That is not what I stand for, and it is not what America stands for.
"I call on ALL Americans to help ease tensions and calm tempers. Thank You."
Will Trump be impeached?
As Democrats hold a majority in the House, the vote is likely to pass.
"We have been asked to turn a blind eye to the criminality, corruption and blatant disregard to the rule of law by the tyrant president we have in the White House," Democratic Representative Ilhan Omar said in the House debate.
"We as a nation can no longer look away."
At least nine Republicans have voted in favour impeachment, but the majority remain loyal to the president.
"Instead of moving forward as a unifying force, the majority in the House is choosing to divide us further... Let us look forward, not backward. Let us come together, not apart," Republican Tom Cole told the House.
He was one of 139 Republicans who last week voted against accepting the result of the 2020 election and Mr Trump's defeat.
Once it has passed in the House of Representatives, the impeachment article 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.
Senate Majority Leader Mitch McConnell said he had not yet decided whether or not he would vote in favour of impeachment.
Mr McConnell has been a key ally for Mr Trump in achieving his legislative agenda and securing a conservative majority on many US courts, including the Supreme Court.
The timeline for a potential trial is not clear but it will not finish before Mr Trump leaves office on 20 January, when Joe Biden will be sworn in as president.
The Senate will not reconvene this week and probably not until 19 January, according to Mr McConnell's spokesman.
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.
In December 2019 he became the third president to be impeached, charged with breaking the law by asking Ukraine to investigate Mr Biden to boost his own chances of re-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 charges by the Senate, where a two-thirds majority required for conviction 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
How many Republicans will vote to impeach Trump?
The third most senior Republican in the House, Liz Cheney, has vowed to back impeachment, saying Mr Trump had "summoned this mob, assembled the mob, and lit the flame of this attack".
"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.
So far, eight more Republicans have voted in favour. Dan Newhouse, a Republican from Washington state, said: "Turning a blind eye to this brutal assault on our Republic is not an option."
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.
On Tuesday, the House passed a resolution calling on Vice-President Mike Pence to help remove Mr Trump using the 25th Amendment - a plan Mr Pence rejected.
Trump's iron grip loosens
With just a week left in his term, it now appears all but certain that Donald Trump will become the first president to be impeached twice.
Unlike his first go through the process, this vote will have the support of at least a handful of Republicans - including Liz Cheney, a member of the party's House leadership team. There is also, unlike January 2020, a chance the Senate has enough votes to successfully convict the president. Majority Leader Mitch McConnell's recent signals of approval are evidence of that.
Of course, the primary consequence of Senate conviction - removal from office - seems of limited relevance with so little time left in the Trump presidency. Democrats, however, view impeachment as a formal way of marking their outrage at the president's behaviour, not just last week, but during his months of challenging and undermining November's election results.
A successful conviction could also result in Trump's being banned from ever holding federal public office again and stripped of the privileges enjoyed by ex-presidents.
That prospect alone, in the minds of Democrats (and, perhaps, some Republicans ready to break with Trumpism), makes impeachment worth the effort.