Biden insists he is staying in the race at pivotal press conference

US President at one point referred to Vice President Kamala Harris as 'Vice President Trump'

US President Joe Biden speaks at a news conference during the Nato summit in Washington. Bloomberg

US President Joe Biden on Thursday used his first press conference since his disastrous debate with Donald Trump two weeks ago to showcase his domestic and foreign policy achievements, and insisted that he would continue his re-election campaign.

“I’m not in this for my legacy,” Mr Biden declared. “I’m in this to complete the job I started.”

Mr Biden, 81, had not held a solo press conference since November, fuelling speculation he was unable to speak extemporaneously on different topics or without the help of a teleprompter.

The press conference was an effort to showcase he is fit to serve another four years in the White House.

His performance was stronger than the recent debate but it was not immediately clear if he had done enough to assuage voter concerns and at one point he referred to Vice President Kamala Harris as "Vice President Trump".

Over the course of nearly an hour, he fumbled a few times in response to about a dozen questions from journalists on a broad range of topics, giving detailed - and sometimes rambling - responses to questions covering domestic and foreign policy issues, as well as his health.

He also fought back against suggestions that he was not in command of his job.

The first question was about him losing support among his own Democratic Party and unions, and about whether Vice President Kamala Harris would be able to beat Trump in a hypothetical head-to-head race.

“Look, I wouldn't have picked Vice President Trump to be vice president if didn't think she wasn't qualified to be president,” he said in one of the night's blunders.

A few minutes later, Trump shared a clip of Mr Biden's gaffe on Truth Social, writing: “Great job, Joe!”

On Thursday, US media reported that the Biden campaign was “quietly” testing with voters a matchup between Ms Harris and Trump.

The news conference comes as several Democrats have called on Mr Biden to step aside after his debate against Trump two weeks ago in Atlanta, Georgia, where the President appeared occasionally bewildered and frequently incoherent.

A number of Democratic politicians and celebrities have publicly called on Mr Biden – who has yet to formally be named the party's nominee – to drop out, arguing he cannot beat Trump in November.

High-ranking Democrat and former House speaker Nancy Pelosi said in an interview it was “up to the President to decide” if he is going to run and warned that “time is running short”.

And in a bruising setback, actor and major supporter of the Democratic Party George Clooney on Wednesday called on Mr Biden to step aside in a New York Times opinion column, warning that not only would the party lose the presidency, it would lose the House and Senate, too.

US voters have long voiced concerns about Mr Biden's age but since the debate it has become an existential challenge to his attempt to secure a second term.

A poll by ABC News, The Washington Post and Ipsos published on Thursday found 85 per cent of US voters said Mr Biden is too old for a second term.

But a defiant Mr Biden appears insistent on staying in the race.

“I think I'm the most qualified person to run for president – I beat [Trump] once and I will beat him again,” he said on Thursday.

He did, however, acknowledge that he needed to “pace” himself more.

Hours before he took the podium, Mr Biden spoke at the end of the Nato summit in Washington, where he accidentally introduced Ukrainian Prime Minister Volodymyr Zelenskyy as “President Putin” before correcting himself several seconds later.

“I’m so focused on beating Putin – we've got to worry about it,” he said.

When Trump was president, he frequently engaged with the press. He called journalists “the enemy of the people” but nonetheless led lengthy, freewheeling briefings that covered a number of topics.

Answering a question about his handling of the war on Gaza, Mr Biden said he was “disappointed” that some of his proposals had not succeeded.

“There's a lot of things in retrospect that I wish I had been able to convince the Israelis to do,” he said.

He also answered questions about the importance of preserving Nato, the war in Ukraine, and keeping Russia and China at bay.

Polls show a tight race between Mr Biden and Trump. The ABC poll shows voters split on Mr Biden and Trump, at 46 per cent to 47 per cent, respectively, if the election were held today.

Mr Biden handily won his party's primary elections this year, making him the presumptive Democratic nominee. Unless he steps aside before the Democratic National Convention in Chicago next month, he will officially be named the party's candidate.

Trump, meanwhile, is consistently polling ahead in swing states.