NATO leaders gather Tuesday in Washington for a summit aimed at showing resolve against Russia and support for Ukraine -- but the meeting is set to be overshadowed by US President Joe Biden's fight for political survival.

The 81-year-old leader will try to use the three days of pomp marking NATO's 75th anniversary to reassure allies over US leadership, and his own ability to govern, as calls grow for him to quit the fight for a second term in office.

Biden has so far defied pressure from some within his own party to step aside, after a disastrous debate against election rival Donald Trump last month threw into stark relief fears he lacks the mental acuity and physical fitness to serve a second term.

"Our allies are looking for US leadership," Biden said in an interview Monday.

"Who else do you think could step in here and do this? I expanded NATO. I solidified NATO."

As doubts swirl over Biden, the rest of the 32-nation alliance is nervously eyeing a potential return to the White House by Trump after elections in November.

On the campaign trail the volatile ex-reality TV star has threatened to blow apart the principle of mutual self-defense that has underpinned NATO since it was founded in the wake of World War II.

But it's not just the United States that faces political questions.

French President Emmanuel Macron arrives after fending off a push by the country's far-right to win power, new British Prime Minister Keir Starmer has his first international outing, and Hungary's premier Viktor Orban flies in after a much-criticized meeting with Russian leader Vladimir Putin.

While they wrestle with the minefield of US politics, NATO leaders will have to show they haven't been distracted from the reality of the battlefield in Ukraine.