Trump remained defiant, vowing to press unfounded claims of fraud as a weary, anxious nation waited for clarity in an election that only intensified the country's deep polarization.
On the fourth day of vote counting, former Vice President Biden had a 253 to 214 lead in the state-by-state Electoral College vote that determines the winner, according to Edison Research.
Securing Pennsylvania's 20 electoral votes would put Biden over the 270 he needs to win the presidency after a political career stretching back nearly five decades.
Biden would also win if he prevails in two of the three other key states where he was narrowly ahead on Friday: Georgia, Arizona and Nevada. Like Pennsylvania, all three were still processing ballots on Friday.
Nationwide, Biden led Trump by 4.1 million votes out of a record 147 million cast.
However, his lead was much smaller in those four contested states: just 84,670 votes out of more than 16 million cast. In Georgia, he led by a mere 3,974 votes.
As Biden's lead grew in Pennsylvania, hundreds of Democrats gathered outside Philadelphia's downtown vote-counting site, wearing yellow shirts reading "Count Every Vote." In Detroit, a crowd of Trump supporters, some armed, protested outside a counting location, waving flags and chanting, "Fight!" Under the banner of "Stop the Steal," Trump supporters planned 62 separate rallies for Friday and Saturday.
Biden planned to deliver a prime-time address on Friday, according to two people familiar with his schedule. His campaign expected that could be a victory speech if television networks call the race for him in the coming hours.
Meanwhile, Trump showed no sign he was ready to concede, as his campaign pursued a series of lawsuits that legal experts said were unlikely to alter the election outcome.
"Joe Biden should not wrongfully claim the office of the President. I could make that claim also. Legal proceedings are just now beginning!" he wrote on Twitter.
Trump earlier leveled an extraordinary attack on the democratic process, appearing at the White House on Thursday evening to falsely claim the election was being "stolen" from him. Election officials across the nation have said they are unaware of any significant irregularities.
Some of Trump's fellow Republicans in Congress said he should tone down his rhetoric.
The Republican National Committee is looking to collect at least $60 million from donors to fund Trump's legal challenges, two sources familiar with the matter said.
In both Pennsylvania and Georgia, Biden overtook Trump as officials processed thousands of mail-in ballots that were cast in urban Democratic strongholds including Philadelphia and Atlanta.
The number of Americans voting early and by mail this year surged due to the coronavirus as people tried to avoid large groups of voters on Election Day. The methodical counting process has left Americans waiting longer than they have since the 2000 election to learn the winner of a presidential contest.
A sense of grim resignation settled in at the White House on Friday, where the president was monitoring TV and talking to advisers on the phone. One adviser said it was clear the race was tilting against Trump, but that Trump was not ready to admit defeat.
The campaign's general counsel, Matt Morgan, asserted in a statement on Friday that the elections in Georgia, Nevada and Pennsylvania all suffered from improprieties and that Trump would eventually prevail in Arizona.
He also said the campaign expected to pursue a recount in Georgia, as it has said it will do in Wisconsin, where Biden won by more than 20,000 votes. A margin that wide has never been overturned by a recount, according to Edison Research.
Georgia officials said on Friday they expect a recount, which can be requested by a candidate if the final margin is less than 0.5%, as it currently is. Biden led by 0.1% as of Friday evening.
In response to the idea that Trump might not concede, Biden spokesman Andrew Bates said in a statement on Friday, "The United States government is perfectly capable of escorting trespassers out of the White House."
The messy aftermath capped a vitriolic campaign that underscored the country's enduring racial, economic and cultural divides, amid a pandemic that has killed more than 235,000 Americans.
If he wins Biden might face a difficult task governing in a divided Washington.
Republicans could keep control of the US Senate, which would enable them to block much of his agenda, including expanding healthcare and fighting climate change.
In Pennsylvania, Biden had a lead of 19,584 votes with 96% of the vote tallied. In Georgia, he was 3,974 votes ahead with 99% of the vote in.
Biden, 77, would be the first Democrat to win Georgia since fellow Democrat Bill Clinton in 1992.
In Arizona, Biden's lead narrowed to 38,455 votes with 94% of the vote counted. His margin in Nevada jumped to 22,657 with 93% of the tally completed.
Pennsylvania, one of three traditionally Democratic states that handed Trump his 2016 victory, had long been seen as crucial to the 2020 race, and both candidates lavished enormous sums of money and time on the "Rust Belt" state.
While many rural areas in Pennsylvania remain in favor of Trump, Democrats are strong in big cities such as Pittsburgh and Philadelphia.
States have historically taken time after Election Day to tally all votes, although in most presidential elections the gap between candidates is big enough that television networks project the winner and the losing candidate concedes before counting formally ends.