WASHINGTON—President Biden held a high-stakes news conference Thursday in which he was forceful and defiant, vowing to pursue re-election and finish the job he started, but he also made some flubs as he tried to slow a stream of Democratic defections that risked turning into a mutiny.

Democrats have continued to break with their 81-year-old standard-bearer following a calamitous debate and public appearances that haven’t reassured colleagues about his fitness for the campaign trail or a second term. Biden has pledged to keep running, but many lawmakers have pushed him to reconsider, with more than a dozen going so far as to publicly call for him to step aside in favor of another candidate.

The news conference was seen as potentially Biden’s last major opportunity to prove he is capable of defeating Donald Trump in November, and he immediately had a misstep.

“Look, I wouldn’t have picked Vice President Trump to be vice president if I didn’t think she’s not qualified to be president,” Biden said, mixing up his opponent’s name with Vice President Kamala Harris . Sticking with his plans to run for re-election, he told reporters he thought he was “the most qualified person to run for president. I beat him once, I will beat him again.”

The news conference along the sidelines of the NATO summit gave the president an opportunity to give extended responses on a number of foreign policy issues, including China and the conflicts in Gaza and Ukraine. Addressing his re-election, Biden said he wasn’t in the race for legacy reasons but to finish the job he started as president, and he offered support for Harris, saying she was “qualified to be president.”

“I’m determined on running but I think it’s important I allay fears by letting them see me out there,” the president said. Biden acknowledged “there are other people who could beat Trump too but it’s an awful lot to start from scratch,” seeming to dismiss the notion of another Democratic candidate emerging at the last minute.

Asked if he would reconsider running if staff gave him data showing Harris would fare better against Trump, Biden said, “No, unless they said there is no way you can win.” He added, whispering: “No one’s saying that. No poll says that.” He added that convention delegates are “free to do whatever they want” but he had “overwhelming support.”

Earlier in the day, Biden had a miscue during an appearance with Ukrainian President Volodymyr Zelensky , accidentally referring to him as Russia’s Vladimir Putin . “Ladies and gentlemen, President Putin,” Biden said, before quickly correcting himself. Zelensky brushed it off, responding, “I’m better.”

Biden was asked about making adjustments to his daily schedule, after it was reported that he was looking at not holding events after 8 p.m. to rest up. “It would be smart of me to pace myself,” Biden said. “Instead of starting a fundraiser at 9 p.m., start it at 8 p.m.,” he suggested.

The president also noted that he had traveled extensively since the debate. “Since I made that stupid mistake in the debate my schedule has been full-bore,” Biden said. He reiterated that he didn’t need to take a cognitive test but would undergo a neurological examination if his doctors advised it.

While many in Biden’s inner circle remain committed to Biden staying in the race, others in the White House and at the campaign are growing increasingly concerned that there is little hope left that the president can salvage his chances for a second term, according to people familiar with the matter. Some who want him to stay are arguing that there isn’t a clear path for how to replace Biden if he decides not to run.

On Thursday, Reps. Hillary Scholten (D., Mich.), Brad Schneider (D., Ill.), Ed Case (D., Hawaii) and Greg Stanton (D., Ariz.) became the 10th, 11th, 12th and 13th House members to say Biden should quit. “For the sake of our democracy, he must pass the torch to a new candidate for the 2024 election,” Scholten, who faces a tough re-election fight this fall, said on social media. More members joined them after the press conference concluded, with Rep. Jim Himes (D., Conn.), Rep. Scott Peters (D., Calif.) and Rep. Eric Sorensen (D., Ill.) saying the president should leave the race.

As of the end of April, he had given fewer interviews and news conferences than any of his recent predecessors, according to data collected by Martha Joynt Kumar , an emeritus professor at Towson University.

Top Biden campaign aides acknowledged in a Thursday memo that the debate was a “setback” but said internal and external polling still shows a tight race in key battleground states. The campaign said winning the blue-wall states of Michigan, Wisconsin and Pennsylvania “is the clearest pathway” to the 270 electoral votes needed to win the presidency.

There is “no indication that anyone else would outperform the president vs. Trump,” the campaign said.

Biden has said he would step aside only if the “Lord almighty” asks him. On Thursday, campaign chair Jen O’Malley Dillon and advisers Mike Donilon and Steve Ricchetti met with Senate Democrats behind closed doors, where they tried to convince Democrats to stay on board.

They came at the invitation of Senate Majority Leader Chuck Schumer (D., N.Y.), who is under pressure from donors and members of his own conference. Schumer has said he supports Biden but has made only brief remarks to that effect amid broad anxiety among Democratic senators.

Senators were mostly tight-lipped coming out of the meeting, saying they had been asked to keep the discussion private. Sen. Richard Blumenthal (D., Conn.) said he still needed to see more data and analytics proving that Biden can win. “My concerns remain,” he said.

On Wednesday, Sen. Peter Welch of Vermont became the first Democratic senator to publicly say Biden should exit the race. Welch said Vice President Kamala Harris would be a capable replacement.

Former House Speaker Nancy Pelosi (D., Calif.) on Wednesday distanced herself from Biden, declining to say if she supported him remaining the nominee.

Other Democrats were subdued. “I want him to do what he thinks is best for him and for our country,” said Rep. Bonnie Watson Coleman (D., N.J.), a member of the Congressional Black Caucus, which has largely backed Biden’s desire to keep running but whose support showed signs of softening.

“I’m riding with Biden no matter what his decision is…I’m with him,” said Rep. Jim Clyburn (D., S.C.), who helped salvage Biden’s campaign in 2020. “I am taking him at his word. I have no idea what’s going on in his head,” he said.

Rep. Adam Smith (D., Wash.), who was one of the first House Democrats to call on Biden to step aside, said the scrutiny on Biden’s performance at the press conference Thursday night just confirms the need to replace him at the top of the ticket as quickly as possible.

“I mean really, we’re having a serious conversation about, you know, can our candidate get through a press conference? The mere fact that we’re having that conversation at this point in the campaign sort of tells you everything you need to know about what needs to be done,” Smith said.

A new Washington Post-ABC News-Ipsos poll found that 56% of Democrats say Biden should end his candidacy, compared with 42% who say he should continue. Asked about who would be best to take Biden’s place, 29% of Democrats and Democratic-leaning independents mentioned Harris, while 7% said California Gov. Gavin Newsom and 4% named former first lady Michelle Obama .

Despite the concerns about Biden, the poll found the president tied with Trump among all registered voters, with 46% each.