Biden predicts victory as counts grow in his favour
Supreme Court denies immediate halt of Pennsylvania count
The Democratic challenger now has 253 of the 270 Electoral College votes needed to clinch the White House under the state-by-state US voting system.
"We're going to win this race," Mr Biden told supporters in Wilmington, Delaware, on Friday night, striking an increasingly confident tone as vote tallies showed his lead extending. He was joined by his running mate, California Senator Kamala Harris.
He said he was on track to win more than 300 Electoral College votes and pointed out that more people had voted for his campaign - over 74 million people - than any US presidential candidate in history.
Biden said Americans had given him a mandate to tackle the coronavirus pandemic, the struggling economy, climate change and systemic racism.
The Democrat - presenting himself as the candidate of unity after a bitterly fought campaign - said it was time to "get the vitriol out of our politics" and "be civil to one another".
"We may be opponents but we're not enemies, we're Americans," said Biden, who did not mention his Republican opponent, Trump.
Biden's appearance had originally been planned as a victory speech, but he opted instead to give a general update on the state of the race as US TV networks cautiously held off declaring him the winner.
The Democrat said he hoped to address Americans again on Saturday.
He's been remarkably consistent throughout the campaign - that's part of his appeal in these chaotic times. He hailed the election results so far as a broad mandate for change, although they're not the resounding repudiation of President Trump for which the Democrats had hoped. Once again he presented himself as a leader who believes in America, who could unify the bitterly divided country - "the purpose of politics isn't total unrelenting warfare", he said.
In the same breath he nodded to his own impatience for an outcome, and presented a stark contrast President Trump's false claims of voter fraud. Watching the ballot tallies is "slow and numbing" he said, but they represent people who "exercise the fundamental right to have their voices heard."
Biden - who ran twice previously for the White House, in 1988 and 2008, without success - would be the oldest president ever inaugurated at 78. If he is declared the victor this weekend, his team is expected to begin its transition process on Monday.
The Secret Service has sent reinforcements to Delaware to beef up Biden's security detail. The Federal Aviation Administration has restricted flights over Wilmington's airspace. However, there is no indication Trump will concede to his opponent in the short term.
"Joe Biden should not wrongfully claim the office of the President," Trump tweeted on Friday afternoon. "I could make that claim also. Legal proceedings are just now beginning!"
Trump has been making unsubstantiated claims of election fraud, spurring some fellow Republicans to speak up that the rhetoric should be toned down.
President Trump is angry and disappointed that more of his allies are not rallying to his side on television or in the streets, according to White House officials on Friday.
He has been watching television, making phone calls to on-the-ground campaign offices, and dividing his time between the Oval Office and the residence.
Several aides did not show up for work and the White House was described as "very empty" with a sombre mood.
Adding to the bad news, it was reported on Friday night that White House Chief of Staff Mark Meadows, 61, had tested positive for coronavirus. That development came on the day the daily US caseload from the virus hit a new record of more than 125,000.
The president has indicated to senior advisers that he will forge ahead with legal challenges to the results, although there is still no firm strategy for such litigation.
One senior outside adviser to the president described his mood on Friday as "somewhere between sullen and hopeful". The source added that Mr Trump "was the last one to think he could win in 2016", despite his top aides telling him that he would.
Biden is leading Trump by more than 4 million votes out of a record 145 million cast. But US presidential election results are decided on a state-by-state basis in the Electoral College, and the contest is much closer in the key battlegrounds.
Biden has 253 Electoral College votes, while Trump has 214. To win the White House, a candidate needs 270.
Meanwhile, a US Supreme Court justice on Friday denied a request by Pennsylvania's Republicans to immediately halt the counting of ballots arriving after Election Day -- referring the challenge to the full court for a ruling on Saturday.
Samuel Alito ordered Pennsylvania in the meantime to continue keeping the late-arriving ballots separate, affirming a decision already made by the state's top elections official.
The last-ditch petition for an emergency injunction -- filed as Democrat Joe Biden solidified his lead and was poised to defeat President Donald Trump -- targeted thousands of ballots.
Most are believed to favor Biden, and Republicans say they should be disqualified under Pennsylvania state law.
As a first step, the party wanted the high court to order ballots arriving after after 8:00 pm on election night to be kept apart from others and prevent them from being tallied.
The concern is that if they are mixed with other ballots, it would render any attempt to disqualify them impossible.
"Given the results of the November 3, 2020 general election, the vote in Pennsylvania may well determine the next president of the United States," the Republicans said.
"It is unclear whether all 67 county boards of elections are segregating late-arriving ballots," the petition added.
Republicans have for months been fighting a state decision to accept mail-in ballots postmarked by November 3 and arriving by Friday. Previously the deadline for acceptance was Election Day itself.
The state supreme court ruled the decision legal and it was then appealed in the federal system.
On October 19 the US Supreme Court, which had a vacant seat, let the state court's decision stand in 4-4 split decision along conservative-liberal lines.
But the high court indicated it could take up the case after the election, and now has nine members after the Trump-nominated conservative Amy Coney Barrett joined in late October.
Trump has explicitly said he wanted Barrett on the court for any election-related case.
Friday's petition appeared more broadly aimed at delaying the eastern state's vote tally from being finalized, which would effectively hand the election to Biden.
A delay could give the high court time to reopen the broader case of the legality of the late ballots.
Even if the court does issue a stay on counting, it might not make a difference. Election analysts say the number of late ballots could be far fewer than Biden's lead over Trump in the state.