As European leaders and top defense officials from 31 Nato countries descend on Washington this week, all eyes will be focused firmly on Joe Biden, whose faltering performance at last month’s debate has added to concerns about the country that some Europeans already described as their “unpredictable ally”.

The US president has hoped that his leadership at the summit will rescue his campaign against Donald Trump amid concerns about his age and mental acuity. In a primetime interview on US television this week, he said: “And who’s gonna be able to hold Nato together like me?… We’re gonna have, I guess a good way to judge me, is you’re gonna have now the Nato conference here in the United States next week. Come listen. See what they say.”

But in private conversations, some European officials and diplomats have expressed concerns about his “shaky” public appearances and worries about the high likelihood of a second Trump term. Several foreign officials questioned whether Biden would remain in the race through the week.

“You can’t just put the genie back in the bottle,” said one European diplomat of the questions concerning Biden’s age. “It is one of the big issues [around the summit].”

Officials who normally focused on security policy said they would pay close attention to Biden’s behaviour during his public appearances at the Nato summit, including a speech in the Mellon Auditorium on Tuesday and then meetings with the other member and partner countries on Wednesday. Some expressed confidence in his team, including Jake Sullivan, US national security adviser, to manage major crises, but said that the question of Biden’s political future had taken a toll.

Several foreign officials said that Biden’s slump in the polls would compound problems from this year’s bruising fight in Congress over the $60.8bn in military aid to Ukraine and make it less likely for the administration to take bold action.

“The issue with his age has become a major concern … a distraction from other real issues [for Nato],” said a European official. One administration official told the Washington Post that the summit has “gone from an orchestrated spectacle to one of the most anxious gatherings in modern times”.

US officials have insisted that Biden is mentally acute, especially pointing at his handling of national security issues such as the Russian war on Ukraine.

A long piece detailing concerns about Biden’s mental state in the New York Times included aides describing his forceful warnings to Benjamin Netanyahu not to launch a massive counterattack against Iran as an example of his good health.

“Look, foreign leaders have seen Joe Biden up close and personal for the last three years,” said a senior administration official. “They know who they’re dealing with and, you know, they know how effective he’s been.”

But that article also said that G7 leaders were concerned about Biden’s physical condition, quoted a European official who said Biden was sometimes “out of it”, and quoted two officials who struggled to say they would put Biden in the same room as Vladimir Putin.

“I’ve heard multiple times [US officials] talking about how he’s very sharp,” a European official told the Guardian. “But he can’t be great just part of the time, he needs to be on his game all of the time.”

Some have gone public with their concerns. “They certainly have a problem,” said Polish prime minister Donald Tusk after last week’s debate. “Yes, these reactions are unambiguous. I was afraid of that. I was afraid … in the sense: it was to be expected that in a direct confrontation, in a debate, it would not be easy for President Biden.”

Especially following the debate, many European diplomats are bracing for a second Trump administration. The former president has openly flirted with the idea of pulling out of Nato and personally harangued members of the alliance who failed to reach a 2% spending benchmark. He has also indicated that he may withhold further aid to Ukraine.

Since early in the campaign, European diplomats have sought to understand Trump’s policies, sending envoys to his campaign or conservative thinktanks like the Heritage Foundation who have produced voluminous briefings about what a second-term Trump administration’s foreign policy could look like.

But Trump’s foreign policy vision remains unclear, they said, subject to his own whims, and will likely be decided at the last minute. (In a surprise on Friday, he disavowed the Heritage Foundation’s Project 2025, often touted as a 900-page road map for his administration’s agenda, saying he “had no idea who they are”.)

“You meet a lot of people who will tell you that they know what Trump is thinking, but no one actually does,” said one European official.

Ahead of the election, officials from Nato countries have sought to “Trump-proof” military aid by having the alliance take over coordination of the Ukraine Defense Contact Group from the US. European countries have also pushed for language in a final Nato communique that would proclaim the “irreversibility” of Ukraine’s accession to the alliance.

“On managing the unpredictability of the US ally … again, it’s not new,” said a European official. “It’s clearly a sentiment which is shared among European allies, that we need to be prepared for the unpredictability of the US ally.”

In a policy brief, Camille Grand, a former Nato assistant secretary general who is now at the European Council on Foreign Relations, said that leaders should prepare to “defend Europe with less America”.

“Even setting aside the outcome of the US presidential election this year and the need to Trump-proof Europe, there is a fundamental and deep trend in US security policy that suggests Europe will have to become less reliant on US support for its security,” he wrote.

Planners want to avoid a repeat of last year’s summit in Lithuania, when Ukrainian leader Volodymyr Zelenskiy tweeted that the lack of a timetable for the country’s accession to Nato was “absurd” after learning of last-minute discussions between other leaders.

“The US team has been making absolutely sure that there wouldn’t be too many or any open issues at the summit to avoid what happened in Vilnius,” Grand said in an interview.

“It’s meant to be a smooth summit and a celebration and an opportunity for Biden to shine, then I guess what the European leaders will be watching in light of the debate is, how is Biden? Is he truly leading? So they will have an eye on him, but I think they will all, at least most of them … rather be in the mood to strengthen him than the opposite.”