US lawmakers are braced for a volatile session to confirm Joe Biden as the next president, as Donald Trump's supporters protest against the result.
A joint session of Congress will count and confirm electoral college votes.
The proceedings to select the president are usually brief and ceremonial.
But some Republicans loyal to President Trump have vowed to support his efforts to overturn the result by formally objecting at the session, in a bid that is almost certain to fail.
For days Mr Trump has also been pressuring Vice-President Mike Pence, who will preside over the session, to block certification of the result.
But constitutional experts say he has no such power and, according to the New York Times and CNN, Mr Pence told Mr Trump he would not intervene to reject the result.
Nevertheless, Mr Trump is gearing up to make a final stand against the certification of his defeat, urging supporters to take to the streets in protest.
Thousands have gathered in Washington DC for a "Save America Rally" at which Mr Trump is due to speak, with counter-protests expected.
Hundreds of National Guard members and police officers have been mobilised in case there is trouble between opposing protesters.
Mr Trump has refused to concede the 3 November election, repeatedly alleging fraud without providing any evidence.
He has also tried to throw doubt on the integrity of Tuesday's Senate run-off vote in the southern, traditionally Republican, state of Georgia. Projections by US TV networks suggest the Democrats have won one of the seats and are neck-and neck-for the second seat.
If the Democrats win both they will gain control of the Senate - something that will help Mr Biden push forward his agenda after he is inaugurated as president on 20 January.
What will happen in Congress?
The two houses of Congress - the House of Representatives and the Senate - will hold a joint session on Wednesday, where they will open sealed certificates from the 50 US states containing a record of their electoral votes.
Under the US system, voters cast their ballots for "electors", who in turn formally vote for the candidates weeks after the election. Mr Biden received 306 votes under the electoral college system, to Mr Trump's 232.
The proceedings are scheduled to begin at 13:00 local time (1800 GMT).
Bipartisan representatives from the two chambers will read out the results on Wednesday and do an official count.
There is a split in the Republican party, with dozens of House Republicans and a smaller group of Senators expected to object to the count from some of the key swing states.
Ted Cruz is leading a group of about a dozen senators calling for a 10-day delay to audit unsubstantiated allegations of election fraud.
Mr Pence - who as president of the Senate is due to declare Mr Biden the winner - earlier said he welcomed this move.
He stopped short of repeating allegations of fraud but his chief of staff said Mr Pence shared what he called "the concerns of millions of Americans about voter fraud and irregularities".
By US election law, Mr Pence's duty in the proceedings is purely administrative, but Mr Trump has urged his vice-president to "come through".
Objections that are endorsed by a member of the House of Representatives and a member of the Senate must be considered by lawmakers in a two-hour debate, followed by a vote.
However, for an objection to be upheld, a majority in both chambers must vote in favour. Republicans hold the majority in the Senate but some of their members have already said they will not contest the results. Democrats are in the majority in the House.
Senate Majority Leader Mitch McConnell has already recognised Mr Biden's victory and asked other Republicans not to object.
What about the protests?
Thousands of supporters of Mr Trump, including some far-right activists, are expected to join the protests in Washington, with crowds gathering near the White House on Tuesday.
"I'm just here to support the president," one protester told Associated Press. "I'm not sure what he can do at this point, but I want to hear what he has to say."
Officials have warned Trump supporters not to carry guns.
Police on Monday arrested the leader of the far-right Proud Boys, charging him with destruction of property related to a previous protest. Enrique Tarrio was later released but a judge ordered him to stay out of Washington.
Mr Tarrio has said on the social media app Parler that the Proud Boys will "turn out in record numbers".
The National Guard has been requested by Washington DC's mayor to assist local authorities. Officials say the troops will not be armed and will be there to help with crowd management and traffic control.
A pro-Trump rally on the National Mall in Washington DC