U.S. President Joe Biden gave most of his aides a mere moment's notice of his decision to quit the race for reelection on Sunday after an agonizing 48 hours poring over polling data which showed his path to victory was closing, two sources said.

Only the day before, Biden was telling many aides that he would continue campaigning to defeat Republican rival Donald Trump in November. In the words of one source, "the message was proceed with everything, full speed ahead".

After digesting the polling data late on Saturday, Biden changed his mind. He gathered his senior White House and campaign team for a call shortly before 1:45 p.m. on Sunday and moments later made his announcement public in a letter to all Americans.

One source said top aides showed Biden internal polling with the jarring news on Saturday night that he was not just trailing in all six critical swing states that could decide the election but also collapsing in places like Virginia and Minnesota where Democrats had not planned on needing to spend massive resources.

Biden was isolating at his home at Rehoboth Beach, Delaware, since testing positive for COVID-19 on Wednesday. Still nursing a cough, he had spent most of the weekend stewing over Democratic pressure to force him to leave the race, aides said.

With him were long-time senior aides Annie Tomasini, Steve Ricchetti and Mike Donilon and a top aide to first lady Jill Biden, Anthony Bernal.

Once resolved, he shared his decision on the call by reading the letter he would soon release.

"He read the letter to us and wanted us to understand his thinking. He said he had wrestled with it over the last 48 hours," said one official.

Immediately after that call, White House Chief of Staff Jeff Zients called senior White House staff together to inform them of the decision.

"This was really closely held," the official said. "It came as a surprise to most White House folks."

Biden and Vice President Kamala Harris had already spoken multiple times during the day on Sunday, a person familiar with their conversations said.