WASHINGTON (AP) — Democrat Joe Biden stood on the cusp of winning the presidency Friday night, three days after Election Day, as the long, exacting work of counting votes widened his lead over President Donald Trump in critical battleground states.

High turnout, a massive number of mail-in ballots and slim margins between the two candidates all contributed to the delay in naming a winner. But Biden held leads in Pennsylvania, Nevada and Georgia, putting him in an ever-stronger position to capture the 270 Electoral College votes needed to take the White House.

There was intense focus on Pennsylvania, where Biden led Trump by more than 27,000 votes, and Nevada, where the Democrat led by about 22,000. The prolonged wait added to the anxiety of a nation facing historic challenges, including the surging pandemic and deep political polarization.

Trump stayed in the White House and out of sight, as more results trickled in and expanded Biden’s lead in must-win Pennsylvania. In the West Wing during the day, televisions remained tuned to the news amid trappings of normalcy, as reporters lined up for coronavirus tests and outdoor crews worked on the North Lawn on a mild, muggy fall day.

Biden, for his part, addressed the nation Friday night near his home in Wilmington, Delaware, and acknowledged the sluggish pace of the count “can be numbing.” But he added, "Never forget the tallies aren’t just numbers: They represent votes and voters.”

He expressed confidence that victory ultimately would be his, saying, “The numbers tell us a clear and convincing story: We're going to win this race.”

Standing alongside his running mate, Sen. Kamala Harris, and against a backdrop of flags, Biden wasn't able to give the acceptance speech his aides had hoped. But he hit notes of unity, seemingly aimed at cooling the temperature of a heated, divided nation.

“We have to remember the purpose of our politics isn’t total unrelenting, unending warfare," he said. "No, the purpose of our politics, the work of our nation, isn’t to fan the flames of conflict, but to solve problems, to guarantee justice, to give everybody a fair shot.”

Trump's campaign on Friday was mostly quiet -- a dramatic difference from the day before, when officials held a morning call projecting confidence and then a flurry of press conferences announcing litigation in key states. But it was touched once again by the coronavirus pandemic.

Chief of staff Mark Meadows contracted the virus, according to two senior White House officials not authorized to publicly discuss private matters. Several other members of the White House staff and Trump’s campaign team also tested positive.

Trump's handling of the pandemic has been the defining issue of the campaign. The president, first lady Melania Trump and several other people in Trump's orbit have fallen ill.

A handful of states remained in play Friday evening — Georgia, North Carolina too early to call along with Pennsylvania and Nevada. In all four states the margins between Trump and Biden were too narrow and the number of ballots left to be counted too great for the AP to declare a victor.

In Pennsylvania, officials were not allowed to begin processing mail-in ballots until Election Day under state law. In Nevada, there were a number of provisional ballots cast by voters who registered on Election Day, and officials had to verify their eligibility. And recounts could be triggered in both Pennsylvania and Georgia.

With his pathway to reelection appearing to greatly 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. It was 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.

He took to Twitter late Friday to pledge further legal action, tweeting that “Joe Biden should not wrongfully claim the office of the President. I could make that claim also. Legal proceedings are just now beginning!"

Trump did claim that he won late on Election Night. He also tweeted that he had "such a big lead in all of these states late into election night, only to see the leads miraculously disappear as the days went by," although it was well known that votes cast before Tuesday were still being legally counted.

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’s erroneous claims about the integrity of the election challenged Republicans now faced with the choice of whether to break with a president who, though his grip on his office grew tenuous, commanded sky-high approval ratings from rank-and-file members of the GOP. That was especially true for those who are eyeing presidential runs of their own in 2024.

Maryland GOP Gov. Larry Hogan, a potential presidential hopeful who has often criticized Trump, said unequivocally: “There is no defense 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.”

But others who are rumored to be considering a White House run of their own in four years aligned themselves with the incumbent, including Sen. Josh Hawley, R-Mo., who tweeted support for Trump’s claims, writing that “If last 24 hours have made anything clear, it’s that we need new election integrity laws NOW.”

Trump’s campaign engaged in a flurry of legal activity, saying it would seek a recount in Wisconsin and had filed lawsuits in Pennsylvania, Michigan and Georgia.

On Friday evening, Supreme Court Justice Samuel Alito approved a GOP request ordering county boards to comply with state guidance to keep the late ballots separate from those received before or on Election Day. However, Alito did not direct election officials to stop counting the ballots, as the Republicans had also sought.

But judges in three states quickly swatted down legal action. A federal judge who was asked to stop vote counts in Philadelphia instead forced the two sides to reach an agreement without an order over the number of observers allowed.

“Really, can’t we be responsible adults here and reach an agreement?” an exasperated U.S. District Judge Paul S. Diamond said during an emergency hearing Thursday evening. “The whole thing could (soon) be moot.”

The Trump campaign said it was confident the president would ultimately pull out a victory in Arizona, where votes were also still being counted, including in Maricopa County, the state’s most populous area. The AP has declared Biden the winner in Arizona and said Thursday that it was monitoring the vote count as it proceeded.

“The Associated Press continues to watch and analyze vote count results from Arizona as they come in,” said Sally Buzbee, AP’s executive editor. “We will follow the facts in all cases.”

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Weissert reported from Wilmington, Delaware. Associated Press writers Colleen Long and Alexandra Jaffe in Washington contributed to this report.