President Donald Trump has yet to admit defeat in the US election. But with a word here, a slip there, he is raising the prospect ever more plainly.

In a tweet Sunday morning, Trump appeared accidentally to acknowledge Joe Biden's victory -- before quickly reversing course to claim he won, and again push unsubstantiated claims of mass electoral fraud while ignoring soaring coronavirus cases.

"He won because the Election was Rigged," Trump tweeted.

The first two words -- coming days after a verbal slip in which Trump said "time will tell" if he remains president -- were immediately seized upon as one more step towards a concession.

US President Donald Trump has refused to concede the November 3 election, and repeatedly said he intends to overturn the result through legal cases -- despite no evidence of mass fraud
US President Donald Trump has refused to concede the November 3 election, and repeatedly said he intends to overturn the result through legal cases -- despite no evidence of mass fraud AFP / MANDEL NGAN

But the president soon made a U-turn, tweeting: "He only won in the eyes of the FAKE NEWS MEDIA. I concede NOTHING! We have a long way to go."

And late Sunday night he said "I WON THE ELECTION!" in a tweet swiftly flagged by Twitter.

President-elect Biden captured 306 Electoral College votes in the November 3 election -- 36 more than needed to win the White House.

Senior federal and state election authorities, including a top cybersecurity agency and 16 federal prosecutors assigned to monitor the elections, have rejected claims of widespread election tampering.

Supporters of US President Donald Trump rally in front of the Cafe Versailles in Miami, Florida on November 3, 2020
Supporters of US President Donald Trump rally in front of the Cafe Versailles in Miami, Florida on November 3, 2020 AFP / Eva Marie UZCATEGUI

Still, Trump continues to insist he will prove fraud and prevail in court.

Meantime, the leaders of nearly every country in the world have congratulated Biden on his victory, reinforcing the notion that almost no one -- in the United States or elsewhere -- is taking the Trump legal challenges seriously.

Those challenges have been nearly universally dismissed by judges as unfounded. On Sunday, Trump insisted that "many" of them had not been filed by his team, and that his "big cases... will soon be filed."

Reacting to Trump's initial tweet, Biden's newly named chief of staff Ron Klain told NBC's "Meet the Press" that it was "further confirmation of the reality that Joe Biden won the election."

A supporter of US President Donald Trump is helped after being assaulted by an unknown assailant at the rally in Washington
A supporter of US President Donald Trump is helped after being assaulted by an unknown assailant at the rally in Washington AFP / Andrew CABALLERO-REYNOLDS

Biden himself met Sunday in his hometown of Wilmington, Delaware with his transition advisors, his spokespeople said.

Some Trump administration officials say privately that they understand that Biden won, but that the president needs time to "process" his loss.

Others, on the outside, speculate Trump may be trying to galvanize his base to back some future commercial or media endeavor or even to support a new run for office in 2024.

US President-elect Joe Biden, seen after attending Mass in Wilmington, Delaware on November 15, 2020
US President-elect Joe Biden, seen after attending Mass in Wilmington, Delaware on November 15, 2020 AFP / ROBERTO SCHMIDT

Until now, the president has refused to cooperate in the shift to a Biden administration -- denying the Democrat both federal funding for transition work and vital briefings by outgoing officials.

Democrats say this could have a damaging impact both on national security and on the grave and mounting challenges posed by the coronavirus, with cases soaring past 11 million since the start of the pandemic Sunday -- one million of those in the last six days alone.

"Joe Biden's going to become president of the United States in the midst of an ongoing crisis," said Klain. "That has to be a seamless transition."

For now, he said, Biden and his team are not even allowed to consult with someone like top government immunologist Anthony Fauci.

"Of course it would be better" if such talks could begin, Fauci told CNN on Sunday, noting that the virus could kill tens of thousands more Americans by the time Biden takes office on January 20.

Former president Barack Obama told CBS's "Sunday Morning" that there was "damage" in Trump's delay in acknowledging Biden's victory.

Millions of people would believe there was fraud -- because the president said so -- in a development corrosive to democracy, he added.

A small but growing number of Republican figures have begun pressing for Trump to concede, including former Trump national security advisor John Bolton, a critic of the president since leaving the administration.

He told CNN it was crucial for more Republicans to persuade Trump that he had lost in a fair election.

Bolton added: "I don't expect him to go graciously. I do expect him to go."

Persuading those who voted for Trump may be a different matter.

More than 10,000 Trump supporters rallied in Washington on Saturday to back his claims of fraud -- massing in Washington's Freedom Plaza before marching to the Supreme Court in a raucous atmosphere.

At least 20 people were arrested after skirmishes erupted, reports said, including four for firearm violations and one for assault on a police officer.

Trump himself made a drive-past of the rally in his armored motorcade, on his way to play golf, smiling through his limousine window to wild cheers.