Host Joe Biden fumbled a June 27 debate, boosting Trump in the polls ahead of a Nov. 5 election that could upend Washington's foreign policy.

NATO leaders also confront political uncertainty in Europe, with paralysis looming in France after gains for left and far right parties, and German Chancellor Olaf Scholz's coalition weakened after a poor showing in European Parliament elections.

The NATO summit, which starts Tuesday, was meant to celebrate the 75th anniversary of a Cold War-era alliance that has found new purpose in opposing Russian President Vladimir Putin's invasion of Ukraine.

“The White House theory was that (the NATO summit) would show Biden's great strengths, which is that he does get this stuff," said Daniel Fried, formerly U.S. State Department's top diplomat for Europe.

"But the debate has knocked everything back. The politics will be overshadowing this."

While much of the attention will focus on Biden amid growing calls within his own party to step aside, the delegates will have a full agenda focused on military and financial aid for Ukraine and offering some pathway toward eventual NATO membership for Kyiv.

NATO chief Jens Stoltenberg, speaking at a pre-summit news conference at NATO headquarters in Brussels, acknowledged that NATO summits always take place within a domestic political context.

But he declared: "What I can do, and NATO can do, is that we can focus on the substance of NATO. And that's exactly what we'll do."

The Biden administration also projected confidence.

A senior Biden administration official briefing reporters on Friday ahead of the summit said allies recognized Biden's work to "reinvigorate the NATO alliance, including expanding it, making it more capable."

"Foreign leaders have seen Joe Biden up close and personal for the last three years. They know who they're dealing with," said the official, who spoke on condition of anonymity.

TRUMP 2.0?

The prospect of a second Trump term alarms many of NATO’s 32 member countries, given his frequent criticism of the transatlantic alliance in and out of office.

Trump has suggested he would not defend NATO members that did not meet the alliance’s defense spending target of 2% of each member's GDP if they came under military attack and has also questioned the amount of aid given to Ukraine.

With the summit taking place just days before the Republican convention that will nominate Trump as the party’s presidential candidate, NATO officials plan to stress to a U.S. audience that its members are stepping up their defense spending.

"The Europeans will have to put a brave face on it and celebrate NATO, celebrate the fact that they've gotten 23 of the 32 members at or above the 2% spending target this year," said Joern Fleck, senior director of the Europe Center at the Atlantic Council.

Asked for comment, a Trump campaign spokesperson said: "We had four years of peace and prosperity under President Trump, but Europe saw death and destruction under Obama-Biden and now more death and destruction under Biden. President Trump got our allies to increase their NATO spending by demanding they pay up."

Despite the concerns, some diplomats pointed out that four years of Trump as president from 2017-2021 did not spell the end of NATO.

"I think we can find ways of working with Trump 2.0 as we worked with 1.0. The approach is different from working with Biden, that's evident. But it's nothing that couldn’t be done. That’s why diplomacy is there," said a senior NATO diplomat in Brussels.

'BRIDGE' TO MEMBERSHIP

For Ukraine, confronting sustained attacks on its cities by Russian forces, the stakes are high.

NATO leaders are expected to endorse an initiative that will see the alliance coordinate arms supplies and training for Ukrainian forces fighting Russia's invasion.

U.S. officials have said Ukraine is also likely to get "good news" at the summit on its appeal for more Patriot air defense systems.

Some members have also signed bilateral agreements with Ukraine and are expected to talk about those commitments in a demonstration of cohesion, according to diplomats.

Ukraine ultimately wants membership of NATO to ward against further future attacks by Russia, but new members have to be approved by all 32 of the alliance's members, some of which are wary of provoking a direct war with Russia.

Moscow sees NATO as a vehicle for U.S. dominance and has accused the alliance of returning to a Cold War mindset.

U.S. officials have said the summit will offer Ukraine a "bridge to membership," which would include the new NATO effort to coordinate arms supplies and training.

Some members want the alliance to make clear Ukraine is moving toward NATO "irreversibly" and are keen for language in a statement emerging from the summit to move beyond the alliance's pledge last year that "Ukraine's future is in NATO."

Ukrainian President Volodymyr Zelenskiy has been invited to the summit and diplomats expect him to attend.