Diplomats and world leaders preparing for next week’s NATO summit are privately expressing acute concern about President Joe Biden’s age, health and ability to win the 2024 presidential election.

These foreign officials largely favor Biden’s reelection and fear that Donald Trump’s return to office would damage the NATO alliance and cripple the war effort in Ukraine. But they have reacted to Biden’s recent debate performance with dismay and fear that Biden may be too frail to defeat Trump and lead a global superpower.

POLITICO spoke with 20 people connected to NATO or the alliance’s upcoming summit over the past month and heard that many allies already had quiet reservations about putting their trust in Biden well before the debate. Now, Biden must convince his counterparts that he’s not only up for the fight but will overcome a political crisis to stay in it.

“It doesn’t take a genius to see that the president is old,” said one official from a European NATO country. “We’re not sure that, even if he wins, he can survive four years more.”

Others went further. “It was painful to watch, let’s be honest,” an EU official said of the debate. “We all want Biden to have a second term to avoid dealing with Trump again, but this isn’t really reassuring.”

Speaking to POLITICO before the U.K.'s change of government on Thursday, a U.K. minister put it most bluntly: “Can the Democrat donors please get their act together and get Biden retired, so we have some chance of a candidate credible for voters?”

Biden already had a tough sell ahead of him at the NATO summit next week, where he was preparing to face questions from allies about America’s commitment to Ukraine. But his catastrophic debate against Trump has turned the gathering into a different kind of assessment of Biden’s physical and political resilience.

Few European leaders have addressed Biden’s age on the record. But Donald Tusk, Poland’s centrist prime minister, offered an extraordinary public expression of alarm after the debate, telling reporters: “They definitely have a problem. The reactions have been unambiguous.”

Mark Gitenstein, the U.S. ambassador to the EU and a longtime Biden adviser, said that any focus on Biden’s age stems from concerns about the election and his ability to secure a second term.

“I have never heard any leader directly or indirectly express a concern to me about his age,” Gitenstein said in an interview. “They’re all worried about the elections, because the elections are close and they’re worried about some of the stuff Trump has said.”

The questions swirling around Biden’s candidacy have turned a summit billed as a celebration of NATO’s landmark 75th anniversary into another stress test for a president whose political future hangs in the balance.

Biden will have to very publicly show off his leadership skills and stamina at the summit in Washington, which begins Tuesday morning and runs through Thursday in what’s forecast to be sweltering heat. As the host, he won’t be able to skip events.

On the summit’s first day, the president will deliver a high-profile speech on NATO’s 75 years at the Mellon Auditorium, where the alliance’s founding charter was signed a few years after World War II. Biden is an advocate for strengthening the alliance, but the message could get lost if he stumbles over key passages or loses his train of thought mid-sentence.

Then on Wednesday — the summit’s busiest 24 hours — Biden will shake hands with leaders of the other 31 member states as well as partner nations. Then he’ll lead a three-hour meeting of the North Atlantic Council featuring the other heads of state and government.

On Wednesday night, Biden will host a dinner of world leaders deep into the night — blowing past his newly self-imposed 8 p.m. work curfew. It will have all the trappings of a state dinner, requiring Biden to be lucid as he pals around with counterparts and discusses sensitive matters away from the cameras.

Biden on Thursday will also attend and lead a marathon session of meetings on topics ranging from the war in Ukraine, to boosting NATO’s deterrence capabilities, to the dynamic security situation in the Indo-Pacific. He’ll have to sit for bilateral sessions with top allies, adding even more to his already packed three-day schedule.

And he’ll cap it all off with a solo news conference, where he is sure to field far more questions about his age and acuity than transatlantic relations.

Among NATO allies’ concerns is how much longer Biden can muster American support for Europe’s defense, especially after what is likely to be a close election against a Republican former president who is skeptical of assisting partners abroad.

“We’re having more conversations about our own defenses since it looks like Trump is coming back,” an official from a NATO country said after the debate.

On top of that, some NATO allies aren’t wholly satisfied with Biden’s leadership, with many saying he’s been too incremental in his approach to providing weapons and giving Kyiv the go-ahead to strike inside Russia.

“Is the U.S. leading or is it just taking part like everyone else?” asked a senior European diplomat in Washington. The grumbles mainly come from hawkish alliance members — typically in Europe’s east — who want military aid to flood Ukraine without limits on Kyiv’s use of it.

Biden’s views on Russia’s invasion of Ukraine are well known, and he repeated many of his talking points during the debate with Trump. He called Russian President Vladimir Putin a war criminal who “wants to reestablish what was part of the Soviet Empire. Not just a piece, he wants all of Ukraine.”

Such talking points and three and a half years as president have given Biden an advantage heading into the summit: He has established solid relationships with most democratic allies, according to three U.S. officials who have been part of various diplomatic engagements.

That’s been true with those whose politics are closely aligned with Biden’s own brand of democratic centrism, like EU Commission President Ursula von der Leyen. He’s also been friendly with ideological foes who share a concern about Ukraine’s fate, namely far-right Italian Prime Minister Giorgia Meloni, and with French President Emmanuel Macron, who openly calls for a bolder European role in its defense and Western troops in Ukraine.

And yet, allies pleased with Biden’s tenure still can’t shake the reality of his age, according to three diplomats.

It’s not so much that they’re upset that he has, at times, skipped out on ceremonial dinners at various summits or left them early. And it’s also not that Biden has been using notecards, speaking more slowly and softly, and moving with a stiffness that’s impossible not to notice, the diplomats said.

They worry about his political standing and reelection chances, knowing that his age is a major political liability.

“It seems to me that’s going to be very tough for him to pursue his campaign and to stay on,” said one senior EU diplomat, who added that while it was up to the Democratic Party whether to replace him, they should be “considering all options.”

Biden’s electoral crisis comes amid political changes in other major NATO powers: British voters installed a new prime minister, the center-left Keir Starmer, just days ago. France holds elections on Sunday that could strengthen the far right and derail Macron’s presidency. German Chancellor Olaf Scholz is already seen by many as a lame duck following June’s EU elections that badly weakened his political coalition.

Ian Bremmer, the president of the Eurasia Group, said he has directly heard fears about Biden’s status from many G7 and European leaders. “Ukraine is still the most important topic,” he said, “but concerns about Biden and growing panic from Europeans about Trump are increasingly distracting the substantive conversations.”