President Joe Biden has mixed up the names of Vice President Kamala Harris and his Republican rival Donald Trump, 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 US for another four-year term.

"The only thing age does is creates a little bit of wisdom if you pay attention," said Biden. 

He 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 November 5 election and keep up with the demands of the job.

AP Explains: How did Biden perform in his make-or-break press conference?

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 about his confidence in Harris.

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 Zelensky as "President Putin" at the NATO summit in Washington, drawing gasps from those in the room.

Moment: Biden mistakenly refers to Zelensky as Putin before correcting himself. – Reuters

Biden's campaign has been on the ropes for two weeks, since his poor debate performance against Donald Trump, his 78-year-old Republican rival.

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."

It's been an excruciating reckoning, and it's not over.

“I think we could lose the whole thing and it’s staggering to me," Democratic Senatpr Michael Bennet of Colorado told CNN in reference to the presidency and both houses of Congress, in what he worries may be a Trump landslide.

Many more kept their newfound alarm about Biden semi-private. Mega-donors froze in the moment.

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.

In highly anticipated press conference, Biden blasts Trump, says GOP opponent has 'no commitment to NATO'. – AP

A White House official said they did not know whether the news conference would shore up support on Capitol Hill.

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.

The news conference gave Biden an opportunity to tout his successes on the world stage and criticize Trump.

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

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

"I think I'm the best qualified person to do the job. To make sure that Ukraine does not fall," he said.

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.

Separately, United Auto Workers union officials met to discuss their concerns with his candidacy, three sources familiar with the matter said, after endorsing Biden in January.

The 400,000-member union has a big presence in industrial states like Michigan that Biden will need to carry to win re-election.

President Biden on European counterparts: "What I hear them say is, you've gotta win". – AP

No Democratic leaders in Congress have called for Biden to end his candidacy, though former House Speaker Nancy Pelosi 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 have called on Biden to drop out, and there were signs that concerns are growing within Biden's campaign operation as well.

Some longtime advisers were reportedly considering ways to convince him to drop his reelection bid, and some campaign staffers reportedly thought he stood no chance of winning the election.

Biden discusses military relations with China, says he has a direct line to Xi. – AP

The Reuters/Ipsos poll found Biden and Trump tied at 40 per cent 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".

Biden says he is determined to seek reelection. – AP

While European leaders who attended the summit were diplomatic about Biden and praised the organisation of the summit, the European press, such as Britain's Daily Telegraph, concluded that "Biden looks finished".

"Slips of tongue happen, and if you always monitor everyone, you will find enough of them," German Chancellor Olaf Scholz said when asked by reporters about Biden confusing Zelensky with Putin.

Newly elected British Prime Minister Keir Starmer, speaking before Biden's gaffes, said he and the president were able to address a number of issues "at pace" during their first meeting.

"He was actually on really good form, and mentally agile - absolutely across all the detail," Starmer told the BBC in an interview.

Spanish Prime Minister Pedro Sanchez said Biden looked "well" and that he attended all summit sessions, unlike other leaders. Italian Prime Minister Giorgia Meloni was less effusive but praised Biden's organisation of "a very good summit".

Other European politicians were less forgiving.

Geert Wilders, whose far-right party won the Netherlands' last election, made fun of Biden, posting a photo of Zelenskiy and Harris on X under the title: "President Putin meets Vice President Trump."

Ukrainians queried by Reuters were mainly sympathetic to Biden for his mix-up of Zelensky with Putin.

"I think he was just tired," Yevhen, a 33-year-old IT specialist in Kyiv who declined to give his surname, said.

But he also worried that such gaffes "could have certain consequences for Ukraine" - given concerns about how Trump, if elected, would treat NATO and Russia's invasion of Ukraine - if the U.S. public stopped trusting Biden

A post-summit press conference failed to convince European media that Biden can rebuild confidence in his mental acuity.

"This was Joe Biden’s chance to win over doubters. He blew it," said a Times of London headline.

Italy's Il Giornale concluded it was the "end of the road for Biden".