US President Joe Biden mixed up the names of Vice President Kamala Harris and his Republican rival Donald Trump on Thursday, but insisted he was pushing ahead with his re-election bid even as more of his fellow Democrats urged him to end his campaign.

Biden, 81, touted his decades of experience on the world stage as he argued that he was uniquely qualified to defeat former President Trump, 78, and lead the U.S. for another four-year term.

"The only thing age does is it creates a little bit of wisdom if you pay attention," said Biden, who is already the oldest person to ever serve as president.

Since his poor performance against Trump in a presidential debate two weeks ago, Biden has faced growing doubts from donors, supporters and fellow Democrats about his ability to win the Nov 5 election and keep up with the demands of the job.

"I've got to finish this job because there's so much at stake," he said. "...There's a long way to go this campaign, and so I I'm just going to keep moving."

He probably did not help his case when he mixed up his vice president and his Republican rival at the outset of the news conference, which lasted nearly an hour.

"Look, I wouldn't have picked Vice President Trump to be vice president if she was not qualified to be president. So start there," Biden said as he responded to a question from Reuters about his confidence in Harris. 

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The president coughed frequently and occasionally garbled his responses at the outset of the news conference, and towards the end his answers frequently trailed off before he had completed his thoughts. At the same time, he delivered detailed responses on issues such as the Israel-Gaza conflict and the need for western countries to produce more military weaponry to counter Russia and China.

That came a few hours after Biden mistakenly referred to Ukrainian President Volodymyr Zelenskiy as "President Putin" at the NATO summit in Washington, drawing gasps from those in the room.

"And now I want to hand it over to the president of Ukraine, who has as much courage as he has determination, ladies and gentlemen, President Putin," Biden said at the NATO summit, drawing gasps from those in the room.

"Going to beat President Putin, President Zelenskiy. I am so focused on beating Putin," Biden said while correcting himself.

A senior Biden-Harris campaign official called Biden's performance the "worst of all worlds. Not good. But not bad enough to make him change his mind". 

At least 16 of the 213 Democrats in the House and one of the Senate's 51 Democrats have appealed publicly to the president to withdraw from the race.

Representative Jim Himes of Connecticut joined that group shortly after the press conference ended. 

"We must put forth the strongest candidate possible to confront the threat posed by Trump's promised MAGA authoritarianism," he said. "I no longer believe that is Joe Biden."

Biden overcame a childhood stutter and has frequently mangled names and misspoken throughout his political career.

Biden said his health is in good shape and that he would take another neurological exam to determine his mental acuity if his doctors recommended it. 

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Biden donor John Morgan called his performance "fantastic", but another donor, likewise speaking on condition of anonymity, said they did not think it would help, given the heightened scrutiny of Biden's verbal slip-ups.

Biden said he needed to "pace myself" a little more and complained that his aides sometimes overscheduled him. "I'm catching hell from my wife," he said.

The news conference gave Biden an opportunity to tout his successes on the world stage at the close of the NATO summit in Washington, where members extended support to Ukraine to combat the invasion that Russian President Vladimir Putin launched in February 2022.

Biden argued Trump would weaken NATO and drive up prices for U.S. consumers by imposing steep tariffs on imported goods.

He took credit for bringing Sweden and Finland into the alliance, and said he brought together 50 nations to support Ukraine.

He also said the Israel-Gaza war must end now and that Israel must not occupy the enclave after the war, adding that both Israel and Hamas had agreed on his ceasefire framework but that there were still gaps to close. 

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No Democratic leaders in Congress have called for Biden to end his candidacy, though former House Speaker Nancy Pelosi on Wednesday declined to say he should stay in the race.

The campaign has commissioned a survey to test how Vice President Kamala Harris would fare if she were to replace Biden as candidate, according to a source with knowledge of the matter. A Reuters/Ipsos poll last week found Harris would fare no better than Biden in a matchup with Trump.

Prominent donors including actor George Clooney have called on Biden to drop out, and there were signs that concerns are growing within Biden's campaign operation as well.

The New York Times reported that some longtime advisers were considering ways to convince him to drop his reelection bid, while NBC News reported that some campaign staffers thought he stood no chance of winning the election.

The Reuters/Ipsos poll found Biden and Trump tied at 40% each. Other opinion polls have found Trump leading Biden, and some strategists have warned that Trump stood a chance of winning reliably Democratic states like New Hampshire and Minnesota.

In a strategy memo, the campaign argued that it has always expected a close election and could win by focusing on three battleground states: Pennsylvania, Michigan and Wisconsin.

If he won those states, along with others considered to be reliably Democratic, he would win 270 electoral votes -- the bare minimum needed to secure the presidency. Biden won 306 electoral votes in 2020.

The campaign characterized other battleground states he won in 2020 as "not out of reach."