It was supposed to be a show of strength -- the leaders of NATO's member countries gathering in Washington, D.C., to display their remarkable unity in the face of some of the most serious threats to the alliance in its 75-year history.

Instead, the spotlight will fall on the summit's host, President Joe Biden, and growing doubts about his capability to serve as president and represent the United States on the world stage for another four years.

But as former President Donald Trump edges ahead in some polls, the looming U.S. election in November has also infused new urgency into some of NATO's key priorities.

Among the anticipated events of the three-day summit are a commemoration event Tuesday at the Mellon Auditorium, the site where the NATO treaty was formally signed in 1949; a bilateral meeting with newly elected U.K. Prime Minister Keir Starmer; a meeting with the EU and NATO's Indo-Pacific partners; and an event with Ukrainian President Volodymyr Zelenskyy and nearly two dozen allies and partners who have signed bilateral security agreements with Ukraine.

On Thursday evening, Biden will hold a rare solo press conference, which he hasn't done since November.

Here are the top storylines to watch:

Biden's next big test

After Biden's debate performance last month ignited panic among Democrats, his campaign has been urgently searching for opportunities to undo damage and prove the president can be an effective leader for the future.

The president himself has set the stakes for the summit remarkably high. He mentioned the alliance six times during his 21-minute interview with ABC News anchor George Stephanopoulos on Friday, saying the event would be "a good way to judge me."

But the optics for Biden's campaign may not be ideal. The summit will mark the 75th anniversary of the alliance, and among its 32 members, Biden is the only head of government who was alive during its founding. The president, 81, will share stages with leaders like France's Emmanuel Macron, Denmark's Mette Frederiksen and plenty of other officials who are about half his age.

In preparation for the summit, officials say Biden has spent hours huddling with Secretary of State Antony Blinken at the White House -- including on the eve of the summit and over the Fourth of July holiday.

Blinken is also expected to be by Biden's side through much of the programming in Washington, according to the officials.

Despite reports claiming U.S. allies have privately questioned Biden's ability to lead, the White House rejected the notion they needed reassurance from the president.

"We're not picking up any signs of that from our allies at all. Quite the contrary. The conversations that we're having with them in advance is they're excited about this summit," White House national security communications adviser John Kirby said on Monday.

Ukraine’s road to membership

Hours before the start of the summit on Tuesday, a barrage of Russian missiles rained down on Ukraine, hitting multiple civilian targets -- including a children's hospital.

The attack -- which left at least 37 people dead, according to Ukrainian officials -- underscored the gravity of the conflict that has indirectly pitted NATO powers against Moscow's aggression.

Before Monday's strike on Ukraine, a senior Biden administration official said that the U.S. would announce new air defenses and military capabilities for the country, adding that the summit would send "a strong signal" to Russian President Vladimir Putin "that if he thinks he can outlast the coalition of countries supporting Ukraine, he's dead wrong."

While Zelenskyy has been pushing for additional air defense systems, he has been critical of NATO's decision to avoid setting a timeline for Ukraine to join its ranks.

A major focus of the Washington summit is expected to be what members of the alliance have branded as Ukraine's "bridge to membership" -- a longer-term effort aimed at steadily moving the country on a path toward becoming a NATO member.

In line with this aim, the alliance is expected to announce additional steps to boost tactical cooperation and force development in the coming days and financial pledges, as well as a host of security agreements between Ukraine and individual NATO members, according to Biden administration officials.

A "Trump-proof" NATO?

Long before the presidential debate in June, NATO observers have been buzzing about efforts to ensure the alliance can stay the course through political headwinds churned up by changes in leadership.

While the alliance's secretary-general, Jens Stoltenberg, is scheduled to step down this fall, much of the attention has been fixed on the U.S. election and what pundits describe as a race to "Trump-proof" the alliance, which the former president has repeatedly threatened to leave. (Trump also said in February he wouldn't protect a NATO nation that didn't contribute enough defense funds and, instead, he'd "encourage" Russia "to do whatever the hell they want.")

"Arguably, despite Russia's continued revanchism, the greatest challenge to NATO today comes from within the alliance -- particularly rising populism and publics that continue to question the value of the alliances -- rather than from adversaries abroad," said Raphael S. Cohen, a senior political scientist at the RAND Corporation.

He argues that addressing that challenge relies on individual members meeting NATO's guidelines calling for each country to commit at least 2% of its GDP to defense spending -- a target 23 of its 32 members are currently meeting.

"It could change not only many American perceptions of the value of NATO but also change European security -- if not global security -- for the better," Cohen said.

But members of the administration who believe a second Trump term in office would do irrevocable harm to the alliance say there's only so much that NATO can do to minimize the impact.

Multiple U.S. officials aligned with Biden told ABC News that although the summit isn't shaping up to be the celebration of the president's foreign policy accomplishments that they anticipated, they hope the summit will draw attention to how a Trump victory in November could damage America's standing internationally.

ABC News' Justin Ryan Gomez contributed to this report.