Biden has been straining to persuade U.S. voters and donors that he's ready for reelection after a stumbling debate performance against Trump. He's been making his case on the campaign trail, in a defiant letter to Democratic lawmakers and now at an international summit that he's still up for four more grinding years in the White House.

"Today, NATO is more powerful than ever--32 nations strong, Biden said as he welcomed leaders at an evening event to mark the 75th anniversary of the alliance. "It's good we're stronger than ever because this moment in history calls for our collective strength," he declared head up and voice strong.

The summit comes at trying moment for Biden's presidency. Several Democratic House members have publicly called on him to quit his campaign. Other lawmakers in private conversations have urged him to step aside, and several high-profile donors have raised concerns about his viability in the race against Trump.

Biden has no shortage of difficult substantive conversations ahead about global security over the course of the three-day NATO summit. But the White House is also looking to display to America that he has the stamina for crowded days and evenings of formal meetings, sideline chats with world leaders, long diplomatic dinners and receptions, and a summit-ending press conference.

Biden on Tuesday announced major steps the U.S. and other NATO countries are taking to beef up Ukraine's overwhelmed air defenses. The U.S., Germany, and Romania are donating additional Patriot batteries, while the Netherlands and other allies will donate additional components to operate a Patriot battery. Italy announced last month it is sending an SAMP/T defense system to help Ukraine.

Other allies, including Canada, Norway, Spain and the United Kingdom will provide other systems aimed at improving Ukraine's defenses.

"This is a pivotal moment for Europe and the transatlantic community, and I might add to the world," Biden declared.

Biden also announced he was awarding outgoing NATO Secretary-General Jens Stoltenberg the Presidential Medal of Freedom. Biden called Stoltenberg, who served in the role for 10 years and extended his tenure fours times, a "consummate professional."

But looming large over the summit were Biden's own political travails with less than four months to go before Election Day.

Several senior administration officials who spoke on condition of anonymity to discuss internal conversations say the president displays a strong grasp of the broader issues -- Russia's invasion of Ukraine, the threat posed by China -- but on specific and incremental actions that countries or groups may take when it comes to these conflicts, Biden has appeared confused at times.

However, the officials say there isn't -- at least not yet -- a crisis in confidence over Biden's general mental state.

German Chancellor Olaf Scholz told reporters Tuesday before departing Berlin for the summit that he did not have any concern about Biden's health. "From the many conversations I have had with the American president, I know that he has prepared this summit very well and very precisely together with us," Scholz said.

The summit will give Biden his first chance to meet face-to-face with new British Prime Minister Keir Starmer. Biden called Starmer last week to congratulate him on his win and plans to host him Wednesday for talks at the White House. Biden is scheduled to meet with Ukrainian President Volodymyr Zelenskyy on Thursday on the sidelines of the summit.

The Labour Party leader had no concerns about Biden's mental acuity during their phone call, according to a spokesman for the prime minister who spoke on condition of anonymity to discuss the private call.

Still, the head of Italy's anti-migrant League Party, deputy premier Matteo Salvini, offered that Biden's "health condition does not seem very good to me." Salvini is a junior member of Premier Giorgia Meloni's right-wing government.

The gathering of the leaders from the 32 NATO countries -- plus Pacific partners Australia, Japan, New Zealand and South Korea, as well as Ukraine -- is expected to be one of Biden's last appearances at an international forum before Election Day and comes before next week's Republican National Convention in Milwaukee.

Biden has sought to spotlight his commitment to the alliance while making the case to voters that Trump would turn his back on NATO if he were to return to the White House.

Trump has repeatedly criticized fellow NATO members who failed to meet an agreed-upon goal of spending at least 2% of gross domestic product on defense. European anxiety was heightened in February when Trump warned NATO allies in a campaign speech that he "would encourage" the Russians " to do whatever the hell they want" to countries that don't meet defense spending goals if he returns to the White House.

Trump has criticized Biden for providing an "endless flow of American treasure" to Ukraine. The Republican more recently has expressed openness to lending money instead and has said Ukraine's independence is important to the United States.

Biden aides have pushed back, noting NATO's announcement last month that 23 of 32 member nations are hitting the alliance's defense spending target this year. Nine member nations were meeting the goal when Biden took office from Trump in 2021.

Biden also has taken credit for the expansion of NATO. Both Finland and Sweden have joined in the aftermath of Russia's February 2022 invasion of Ukraine.

"That's not by accident," White House national security spokesman John Kirby said. "That's because of leadership. That's because of a constant stewardship of the alliance and other partnerships around the world. The president's record speaks for itself."

NATO is expected to announce details of Ukraine's pathway to membership into the alliance during the summit. NATO, which is built around the foundational agreement that an attack on one member is an attack on all, has maintained it will not bring Ukraine into the fold until after the conflict with Russia ends.

Kirby said leaders also will discuss efforts to create a coordination center in Germany to help train, equip and coordinate logistics for Ukraine forces for its expected eventual accession into NATO.

"It could be easy to look around in Washington this week and see the unity, the resolve, the capabilities of NATO today and say, that's just the natural order of things," White House national security adviser Jake Sullivan said, "But it isn't just the natural order of things. It's taken work. It's taken strategy."

As for Biden's health, Ian Brzezinski, a senior fellow at the Washington think tank the Atlantic Council, said he needs to use the summit to "significantly reverse the impression" that he left with his poor debate performance.

"This is an immense opportunity for him to lead with vigor and energy, to underscore his commitment, the administration's commitment, for that matter Congress's commitment to the alliance and to underscore that he brings to the table the resolve that has made NATO so successful," Brzezinski said.

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Associated Press writers Fatima Hussein and Matthew Lee in Washington, Kirsten Grieshaber in Berlin, Jill Lawless in London and Nicole Winfield in Rome contributed to this report.