Democrat Joe Biden has narrowly overtaken president Donald Trump in the vote counts in Georgia and Pennsylvania, with the presidency hinging on the outcome of tight contests in key battleground states.
Neither candidate has reached the 270 Electoral College votes needed to win the White House, though Biden has the advantage after eclipsing Trump in Wisconsin and Michigan, two crucial Midwestern battleground states.
That left both campaigns increasingly focused on developments in Pennsylvania and Georgia, where Biden had an advantage of more than 900 votes on Friday.
He has now moved ahead of the incumbent president in Pennsylvania although the race is still too close to call.
It could take several more days for the vote count to conclude in some states, allowing a clear winner to emerge.
With millions of ballots yet to be tabulated, Biden has already received more than 73 million votes nationally, the most in history.
As Americans entered the third full day after the election without knowing who won the race, anxiety about the outcome was building.
With his pathway to reelection appearing to narrow, Trump was testing how far he could go in using the trappings of presidential power to undermine confidence in the vote.
On Thursday, he advanced unsupported accusations of voter fraud to falsely argue that his rival was trying to seize power in an extraordinary effort by a sitting American president to sow doubt about the democratic process.
“This is a case when they are trying to steal an election, they are trying to rig an election,” Trump said from the podium of the White House briefing room.
Biden spent Thursday trying to ease tensions and project a more traditional image of presidential leadership.
After participating in a coronavirus briefing, he declared that “each ballot must be counted.”
“I ask everyone to stay calm. The process is working,” Biden said.
“It is the will of the voters.
“No one, not anyone else who chooses the president of the United States of America.”
Trump showed no sign of giving up and was back on Twitter, insisting the “US Supreme Court should decide!”
Maryland Republican governor Larry Hogan, a potential presidential hopeful who has often criticised Trump, said unequivocally: “There is no defence for the President’s comments tonight undermining our Democratic process.
“America is counting the votes, and we must respect the results as we always have before.”