Follow our coverage of the debate here
Incumbent Joe Biden won the Democratic primary elections earlier this year, making him the presumptive nominee for US president, but Thursday night's debate has cast doubt over his future prospects.
With a little more than four months to go until the November 5 vote, he has not yet been named the official candidate for his party. That is scheduled to happen during the Democratic National Convention, which will take place in Chicago in mid-August.
Mr Biden's dismal performance during Thursday's first debate against his presumptive Republican rival Donald Trump drew calls from his own party to step aside and allow another candidate to run instead.
“I wish Biden would reflect on this debate performance and then announce his decision to withdraw from the race, throwing the choice of Democratic nominee to the convention,” Nicholas Kristof, a New York Times columnist wrote on X, before naming possible candidates.
Maria Shriver, a prominent member of the party and part of the politically influential Kennedy family, put it more bluntly. “There’s panic in the Democratic Party,” she wrote.
It would be highly unusual for a sitting US president to step aside, especially this late in the election cycle. Mr Biden has given no indication that he would be willing to end his campaign, and some political analysts have pointed to his pride and stubbornness.
There is no clear front-runner for the President's possible replacement, but here are the most prominent possible candidates:
Kamala Harris
Mr Biden's Vice President, Kamala Harris, is in many ways the most obvious choice: if he were to resign now, she would automatically become president.
If Mr Biden were to drop out of the race and recommend that she be nominated, the party is likely to weigh that into their decision and rally behind her.
After the debate, Ms Harris defended the President, telling CNN: “I’m not going to spend all night with you talking about the last 90 minutes when I’ve been watching the last three and a half years of performance.”
But polls show Ms Harris, 59, is unpopular among US voters and she is likely to struggle against Trump. Observers also say she has found it difficult to carve out a concrete role for herself in the Biden administration.
Gavin Newsom
The Governor of California is another possible replacement for Mr Biden. He was present in CNN's “spin room” during the debate, delivering sharp criticisms of Trump and defending the President.
The 56-year-old leader of the most populous US state is a well-known figure in the party.
But in an interview after the debate, he dismissed suggestions that he would replace Mr Biden as the Democratic candidate.
“This is a President of the United States running for re-election – he's our guy,” Mr Newsom said.
“And that's part of the diversion tactic of the right – to continue to sort of muddy the waters on all of that, and that nonsensical speculation.”
Gretchen Whitmer
The Michigan Governor is a rising star in the Democratic party, and the 56-year-old was also on the shortlist of candidates for Mr Biden's vice president in 2020.
She is co-chairwoman of Mr Biden’s re-election campaign.
Secretary of Transportation Pete Buttigieg, Pennsylvania Governor Josh Shapiro, Illinois Governor JB Pritzker and California Representative Ro Khanna have also been floated as potential candidates.
