Responding to Mr Biden’s endorsement, Ms Harris said: “I am honoured to have the president’s endorsement and my intention is to earn and win this nomination.”
But Mr Obama, the former president, along with senior Democrats Chuck Schumer, Nancy Pelosi and Hakeem Jeffries, praised Mr Biden’s decision without openly endorsing Ms Harris on Sunday evening.
Writing on his blog, Mr Obama said: “I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges.”
That suggested he did not consider Ms Harris outstanding enough to immediately green-light her elevation to the top of the ticket. Several weeks remain until the Democrat National Convention begins on Aug 19.
A person familiar with Mr Obama’s thinking told the New York Times he was staying above the political fray as he had in the 2020 primaries, and was ready to “help unite the party once we have a nominee.”
There is widespread scepticism about Ms Harris’ ability to appeal to blue-collar voters in the battleground Midwestern states, where Mr Biden was able to perform strongly in 2020.
Gretchen Whitmer, the Michigan governor, and Gavin Newsom, the California governor, had their names floated as potential replacements to Mr Biden following his disastrous debate performance against Donald Trump in June 2024.
However, sources close to both figures told CBS News that both were preparing to stand down and would not seek to challenge Ms Harris at the convention in Chicago. If they had, it would have caused a messy fight on the convention floor.
Sources close to Joe Manchin, the 76-year-old West Virginia senator, told US media that he may seek to challenge Ms Harris for the nomination. Having left the Democrats to sit as an independent this year, he would have to register with his old party before doing so.
Ron Klain, Mr Biden’s former chief of staff and an ally of the Democrat for decades, urged the party to unify behind Ms Harris.
“Now that the donors and electeds have pushed out the only candidate who has ever beaten Trump, it’s time to end the political fantasy games and unite behind the only veteran of a national campaign – our outstanding VP, Kamala Harris,” he said.
Major Democratic donors have also lined up behind Ms Harris, amid reports that they were refusing to sign campaign cheques as Joe Biden fought to maintain his election campaign in the weeks following his disastrous debate with Donald Trump.
A source told CNN: “President Biden made the right decision. Now big donor money will start flowing again. His selfless act will save our republic.”
Another source close to Democratic donors said: “It’s an avalanche of money.”
Tory Gavito, president of Way to Win, a group for donors aligned with Democratic causes, said she believed that Ms Harris was the right person to head into the election.
“We are ready to lock in and support vice-president Harris in her new role,” she said. “She’s had a stellar track record in all of her political positions.
“She has refined her case for voting rights, her case for reproductive freedoms, her ability to govern on the economy and she’s ready.”
If no major candidate emerges as a potential challenger to Ms Harris, then attention will quickly turn to who will join her ticket as running mate.