WASHINGTON — Growing skepticism about President Joe Biden’s re-election chances has European leaders heading to the NATO summit in Washington confronting the prospect that the military alliance’s most prominent critic, Donald Trump, may return to power.

NATO — made up of 32 European and North American allies committed to defending one another from armed attack — will stress strength through solidarity as it celebrates its 75th anniversary during the summit starting Tuesday.

Event host Biden, who pulled allies into a global network to help Ukraine fight off Russia’s invasion, has called the alliance the most unified it has ever been. But behind the scenes, a dominant topic will be preparing for possible division.

At the presidential debate, Biden asked Trump: “You’re going to stay in NATO or you’re going to pull out of NATO?” Trump tilted his head in a shrug. Biden’s poor debate performance set off a frenzy about whether the 81-year-old president is fit for office or should step aside as the Democratic presidential candidate.

Even before the debate, European governments were deep in consultations on what they could do to ensure that NATO, Western support for Ukraine and the security of individual NATO countries will endure should Trump win back the presidency in November.

This week’s summit, held in the city where the mutual-defense alliance was founded in 1949, was once expected to be a celebration of NATO’s endurance. Now, a European official said, it looks “gloomy.”

Rachel Rizzo, a senior fellow on NATO with the nonpartisan think tank the Atlantic Council, says she has a blunt message for Europeans:

“Freaking out about a second Trump term helps no one.” For allies at the summit, she said, the key will be resisting the temptation to dwell on the details of unprecedented events in U.S. politics and put their heads down on readying Western military aid for Ukraine and preparing for any lessening of U.S. support.

One of Trump’s former national security advisers, John Bolton, says Trump in a second term would work to get the United States out of NATO. Congress passed legislation last year making that harder, but a president could simply stop collaborating in some or all of NATO’s missions.

Trump’s campaign did not immediately respond to an email seeking comment. An initiative likely to be endorsed at the summit is NATO taking more responsibility for coordinating training and military and financial assistance for Ukraine’s forces.

Europeans say NATO countries are coordinating statements on Ukraine for the summit to make clear, for example, that additional Russian escalation would trigger substantial new sanctions and other penalties from the West. That’s even if the U.S., under Trump, doesn’t act.