In a critical press conference meant to make or break his presidential campaign, Joe Biden spiritedly defended his foreign policy record even as he faced a barrage of questions on his mental fitness and, in another gaffe, mistakenly referred to Kamala Harris as “vice-president Trump”.

Biden offered extensive remarks on thorny foreign policy issues including competition with China and the Israel-Hamas war, in which he said he had warned Israeli prime minister Benjamin Netanyahu away from an occupation of the Gaza Strip.

He said he was directly in contact with Xi Jinping to warn him not to offer further support for Russia’s war in Ukraine, but not with Vladimir Putin, whom he said: “I have no reason to speak to him right now.”

But Biden, who is running to be president until January 2029, fielded an equal number of questions during the press conference on his mental fitness, an issue that has loomed over his campaign since a faltering debate performance against Donald Trump that he called “that dumb mistake”.

Ultimately, it was a performance that supporters will probably say shows he is capable of handling his responsibilities as commander-in-chief, but unlikely to convince those already in doubt about his mental fitness that he can serve another four years in office.

Biden, 81, insisted he would stay in the race despite calls from some in his party to drop out and to allow another figure, including Harris, run in the November election.

Shortly after he finished speaking, Connecticut congressman Jim Himes, the top ranking Democrat on the House intelligence committee, called on Biden to step down from the campaign, writing on X: “We must put forth the strongest candidate possible to confront the threat posed by Trump’s promised MAGA authoritarianism. I no longer believe that is Joe Biden.”

Appearing later on CNN, Himes said: “Imagine that three months from now, we get another performance like there was in the debate, right before the election. Do you want to take that risk? I don’t.”

Two more congressional Democrats also called on Biden to step aside, bringing the total to 17. Representative Scott Peters of California said, “The stakes are high, and we are on a losing course,” while Representative Eric Sorensen of Illinois said that Biden should “put country over party”.

Wrapping up a summit of the 32-member bloc in Washington DC, Biden said: “I’ve not had any of my European allies come up and say, ‘Joe, don’t run’. What I’ve heard them say is, ‘You’ve got to win’.

“If I slow down and can’t get the job done that’s a sign I shouldn’t be doing this,” he said. “But there’s no indication of that. None.”

Biden said he wouldn’t leave the race unless polls showed him that he had no chance of winning against Trump, even if they showed that Harris’s chances in the election were better than his own.

Nonetheless, he said Harris was qualified to be president as well, although he misnamed her in the endorsement. “I wouldn’t have picked vice-president Trump to be vice-president, if she’s not qualified to be president,” he said.

That gaffe was compounded by the fact that he had introduced the Ukrainian president, Volodymyr Zelenskiy, as “President Putin” just hours earlier, before correcting himself and saying “we’re going to beat Putin”.

Biden initially used the final Nato summit press conference as something of a stump speech, brandishing his national security record in supporting Ukraine against Russian aggression and saying that the November vote was “much more than a political question … It’s a national security issue.”

He then turned to his record on the economy, border security and his efforts to broker a peace in the Israel-Hamas war to bolster his case for his campaign in November.

Biden spoke for 58 minutes, including 50 minutes of unscripted question-and-answer. He appeared most comfortable and cogent as he discussed thorny foreign policy questions.

“Don’t make the same mistake America made after [Osama] bin Laden,” he said he told Netanyahu, as he sought to ward off a potential occupation of the Gaza Strip. “There’s no need to occupy anywhere. Go after the people who did the job.”

He also indicated that European countries were prepared to cut their investments in China if Xi continued to “[supply] Russia, with information and capacity, along with working with North Korea and others, to help Russia in armament”.

But at times he got lost in the weeds. Asked about reports that he had asked his schedule to be moved up, he said: “I’m not talking about, and if you’ve looked at my schedule since I, since I made that stupid mistake in the campaign, in the debate. I mean, my schedule has been full bore.”

“Where’s Trump been?” he continued. “Riding around on his golf cart? Filling out his scorecard before he hits the ball?”