The 538 electoral college representatives prepared to cast their votes on Monday, despite departing President Donald Trump's continued refusal to concede to president-elect Joe Biden.
The vote represents an official landmark in upholding the November 3 election result and the penultimate step in finalising Mr Biden's victory against Mr Trump by 306-232 electoral college votes.
While more than 155 million Americans voted in the 2020 election, it is the meetings of electoral college representatives from the 50 US states and the District of Columbia that select the president and the vice president.
The process, which dates from the 19th century, mandates that those electors meet in their state capitols and formally vote for the country's leaders.
There is nothing in the US Constitution that requires these electors to vote for the winner of the election.
But not doing so is rare, and would subject an elector to a fine and render his or her status as "faithless", under a law held by the Supreme Court.
Most of the electors will be at their local state houses, with the exception of Nevada, Utah and Colorado.
Their representatives will break a 200-year tradition and meet online because of the pandemic.
After the vote, the ballots will be shipped by the US Postal Service to Washington, where they must arrive at Congress no later than December 23.
There they will be counted by the Senate in a special session presided over by departing Vice President Mike Pence on January 6.
That represents the last step before the inauguration of Mr Biden on January 20.
While Mr Trump can cause delays and objections through House members on January 6, he will officially run out of options to dispute the election after Monday’s vote.
He lost two legal bids at the Supreme Court and 59 at local courts.
Mr Trump said that he would leave the White House if the electoral college representatives did not declare a winner on Monday. "Certainly I will, and you know that," he said on November 27.
But he may never concede and it is unclear if he will attend Mr Biden's inauguration. He pledged more legal battles.
“No, it's not over. We keep going and we're going to continue to go forward. We have numerous local cases,” he told Fox News.
Mr Trump evaded the question of whether he would attend the inauguration.
"I don't want to talk about that," he said.
Mr Trump's supporters clashed with anti-Trump groups in Washington DC at the weekend.
At least four people were stabbed at night time in the downtown area and more than 23 were arrested.
The city police department said two of its officers were injured in the clashes on Saturday.
It said it charged 10 people with offences including assaulting police officers and rioting.