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President , in his latest effort to vote tallies showing him headed for defeat in the tense US election, warned challenger on Friday against "wrongfully" claiming the presidency.

Three days after the US election in which there was a record turnout of 160 million voters, a winner had yet to be declared.

However, incomplete returns from key states showed Biden almost certain to make Trump a one-term president. The Democrat was ahead in Arizona, Nevada and even Georgia, a historically Republican state where Biden was so close to victory that authorities announced a recount.

In the big prize of Pennsylvania, Biden's lead inexorably grew as final batches of ballots were added up. By late Friday he had an advantage of almost 20,000 votes over Trump, although that was still within the narrow margin that would likely require a recount.

Should Biden, 77, be confirmed to have won Pennsylvania, he will automatically cross the threshold for winning the presidency.

Biden was set to give an address in his hometown of Wilmington, sparking speculation that he planned to declare victory. But with US television networks holding off from naming him the formal winner, he might change his plans to something more modest.

Trump has several times prematurely named himself the winner, refusing to accept the data showing Biden headed for victory.

In his latest broadside, he warned on Twitter that "Joe Biden should not wrongfully claim the office of the President. I could make that claim also."

Trump's campaign has filed lawsuits around the country alleging fraud, but no evidence has been produced to back up supposed incidents of ballot tampering or other significant incidents.

However, in an apparent scaling back of earlier comments accusing the Democrats of vote rigging and "stealing" his victory, Trump issued a somewhat milder written statement Friday, warning "about the integrity of our entire election process."

Mail-in ballots

In another tweet, Trump complained that he had "such a big lead" on election night, "only to see the leads miraculously disappear" later in the week.

Trump has now said falsely several times that ballots are being either invented for Biden or stolen from him.

But the unprecedented attacks on US election integrity by a president ignore a simple fact about the different types of ballots cast.

Votes cast in person on Election Day largely favored Trump and were often counted first, giving him an early lead. However, later counting turned to the avalanche of votes mailed in by Americans who did not want to go to crowded polling stations in the middle of the coronavirus pandemic.

The mail-in versions overwhelmingly favored Biden, because he had encouraged Americans to use the method. Trump, by contrast, frequently scoffed at the dangers of Covid-19 and railed against mailed ballots.

As a result, his initial advantages in the vote tallies quickly shifted to a surge of pro-Biden votes, erasing his lead.

Biden's unity message

Biden currently has at least 253 electoral votes from states he has already won. Pennsylvania, and its 20 allotted votes, would be enough to vault Biden past the magic number of 270 votes in the 538-member Electoral College, which determines the White House race.

If Biden's victory is confirmed, the former senator from Delaware would be sworn in on January 20, 2021 as the 46th president of the United States.

His running mate, California Senator Kamala Harris, 56, would be the first Black woman to become vice president and the first of South Asian descent.

House Speaker Nancy Pelosi, the top Democrat in Congress, said Biden would help bring the country together after Trump's polarising presidency.

"Joe Biden is a unifier because he is determined to bring people together," Pelosi said.

'Far from over'

Amid growing concerns about the potential for unrest if Trump refuses to concede, attention was focused on the reaction of his Republican Party.

Pennsylvania Republicans asked the US Supreme Court Friday to halt the counting of late-arriving ballots in the state.

The last-ditch appeal for an emergency injunction asked the court to freeze the processing of thousands of mailed ballots - most believed to be favoring Biden - that arrived after Election Day, which Republicans say should make them disqualified.

Several prominent Republicans meanwhile rallied behind Trump.

"Far from over," tweeted Representative Kevin McCarthy, the leader of the Republican minority in the House. "Republicans will not back down from this battle."

"I think everything should be on the table," Senator Lindsey Graham said when asked if Pennsylvania's Republican-led legislature should refuse to certify the results.

Other top Republicans denounced Trump's comments including Utah Senator Mitt Romney, the only Republican member of the Senate to vote to convict the president at his impeachment trial earlier this year.

"He is wrong to say that the election was rigged, corrupt and stolen - doing so damages the cause of freedom here and around the world, weakens the institutions that lie at the foundation of the Republic, and recklessly inflames destructive and dangerous passions," Romney said.

Powerful Senate Republican majority leader Mitch McConnell had a nuanced statement. 

"Here's how this must work in our great country: Every legal vote should be counted," McConnell said. "Any illegally-submitted ballots must not.

"All sides must get to observe the process. And the courts are here to apply the laws & resolve disputes."

Agencies contributed to this report.