U.S. President Joe Biden pledged to forcefully defend Ukraine against Russia's invasion at the NATO summit in Washington, using the global stage to try to show allies at home and abroad that he can still lead.

Biden, 81, has endured 12 days of withering questions about his fitness for office as some of his fellow Democrats on Capitol Hill and campaign donors fear that he will lose the Nov. 5 election after a halting debate performance on June 27.

"(Vladimir) Putin wants nothing less, nothing less, than Ukraine's total subjugation ... and to wipe Ukraine off the map," Biden said in his welcome to NATO member states to the summit, referring to the Russian president. 

"Ukraine can and will stop Putin."

The White House is hoping he can turn the page on a difficult period in his presidency with his highest profile policy speech since the debate, although some diplomats at the summit said the damage was hard to erase.

Biden was framed by the gilded walls of the federal hall where the treaty creating NATO was signed, his speech bookended by stirring musical performances by the U.S. Marine Corp band.

"Today NATO is stronger than it's ever been in its history," he said.

Biden has rebuffed calls to step aside in his race against Republican Donald Trump, 78, vowing to beat him in November. So far, he has maintained the public support of most of his party's elite.

The U.S. president has made restoring traditional alliances abroad the centerpiece of his foreign policy after Trump challenged allies as part of an "America First" approach. The election winner in November could have a substantial impact on the future of NATO, Europe and the rest of the world.

"We don't see how he can come back after the debate," said one European diplomat, who dismissed Tuesday's speech as evidence of Biden's endurance because it was scripted. "I can't imagine him being at helm of the U.S. and NATO for four more years."

Trump has suggested that, given a second term, he would not defend NATO members if they came under military attack and did not meet the alliance's defense spending target of 2% of their annual GDP. He has also questioned the amount of aid given to Ukraine in its battle against Russia's invasion.

Biden closed his remarks by surprising NATO Secretary-General Jens Stoltenberg with the Presidential Medal of Freedom, clasping the highest U.S. civilian award around the Norwegian politician's neck and crediting him with reviving the 32-member alliance.