Moments after President Biden assured Volodymyr Zelensky that he could count on United States support for as long as it took, the Ukrainian leader used the opportunity to speak not only to NATO allies but also to an audience thousands of miles away.
“I understand that it’s all your money,” Mr. Zelensky said, addressing Americans directly. “You spend this money for our lives.”
Despite Mr. Biden’s repeated promises of staying by Ukraine’s side in its war against Russia, questions about the shelf life of support among American people and lawmakers hung over the summit of Western allies. Even as the U.S. president was giving a long-term commitment, a group of far-right Republican lawmakers in Washington was pushing legislation that would scale back aid to Ukraine, exposing fractures in the Republican Party and raising doubts about its commitment should it capture the White House next year.
The two G.O.P. candidates leading in polls, Donald J. Trump and Ron DeSantis, have also expressed reservations about maintaining the war as a priority for the United States, fueling concern among some Western allies and injecting the American electoral cycle as a major element in Ukraine’s prospects for victory.
At the NATO summit, Mr. Biden was intent on addressing those doubts, vowing to continue to rally the alliance in support of Ukraine and speaking to his domestic audience back home, preparing Americans for a prolonged standoff with Russia. During a speech from Vilnius University, in the Lithuanian capital, he compared the plight of Ukraine to the Cold War struggle for freedom in Europe, a fight that had the overwhelming support of both the Democratic and Republican Parties.
“We will not waver,” Mr. Biden said, a message echoed by most NATO leaders. “I mean that. Our commitment to Ukraine will not weaken.”
Still, some leaders openly questioned just how long Kyiv could count on robust U.S. support.
Ukraine needed to make military progress more or less “by the end of this year” because of the coming elections in the United States, President Petr Pavel of the Czech Republic warned on the first day of the summit. By next year, he suggested, there could also be “another decline of willingness to massively support Ukraine with more weapons.”
Ben Wallace, Britain’s defense minister, went so far as “providing a slight word of caution” that Ukraine should express more appreciation to allies for sending tens of billions of dollars in aid to Kyiv.
“Sometimes you’ve got to persuade lawmakers on the Hill in America,” Mr. Wallace said. “You’ve got to persuade doubting politicians in other countries that, you know, that it’s worthwhile.”
(In a stern reply to Mr. Wallace, Mr. Zelensky later told reporters, “He can write to me about how he wants to be thanked.”)
Even as President Recep Tayyip Erdogan of Turkey expressed optimism about collaborating with the United States at future NATO summits, he joked about the uncertainty of future U.S. leadership. “With the forthcoming elections, I would also like to take this opportunity to wish you the best of luck,” Mr. Erdogan told Mr. Biden, prompting the American president to laugh and reassure him that he would be meeting him again in the years ahead.
Mr. Biden was pressed again on Thursday about the reliability of the United States’ commitment during a joint news briefing with President Sauli Niinistö of Finland. Asked by a European journalist about “political volatility” in Congress, Mr. Biden said “there’s no question there’s overwhelming support from the American people, there’s overwhelming support from the members of Congress.”
“No one can guarantee the future, but this is the best bet anyone could make,” Mr. Biden added.
But the concerns expressed by those leaders appeared to have some grounding, given Republican skepticism.
“I’m of course concerned about the leadership,” said William Taylor, a former ambassador to Ukraine in the Bush and Obama administrations. “American leadership on this issue is going to be key, and it will have to continue to be bipartisan.”
Mr. Biden’s aides say they believe his ability to build support for Ukraine both domestically and overseas will be one of the lasting achievements of his presidency. He has sold himself as someone who can repair the divisions deepened by his rivals, and on the campaign trail he is expected to emphasize his consensus-building in the halls of Congress and on the global stage during what he has described as an inflection point for the world.
Turkey’s decision to end a block on Sweden’s entrance to NATO and Mr. Zelensky’s declaration that the summit had given Ukraine a “significant security victory” will probably help Mr. Biden’s case. But many American voters remain unconvinced, particularly about his economic record, fueling his low approval numbers.
Over the past year, Mr. Biden has attempted to frame the economic hardship that comes with aiding Ukraine as a cost of defending democracy.
But some support among the public has wavered at times as Americans faced soaring consumer prices and Europeans grappled with an energy crisis after cutting their reliance on Russian gas.
The Consumer Price Index reported on Wednesday that U.S. inflation had cooled slightly in June, providing an assist to Mr. Biden’s pitch. Federal Reserve officials are still assessing, however, just how long the trend will last. Consumer price rises remain above the rate of increase from before the pandemic.
A recent Reuters-Ipsos survey found a sharp rise in support among the American public for helping Ukraine’s effort to defend itself against Russia. The survey found that 81 percent of Democrats, 56 percent of Republicans and 57 percent of independents favored supplying U.S. weapons to Ukraine. The poll also found that a large majority of Americans were more likely to support a presidential candidate who would continue to provide military aid to Kyiv.
“This is a good debate to have,” Mr. Taylor, the former ambassador, said. “The American people deserve to participate in the debate about the support for Ukraine and the opposition to Russia’s invasion.”
Mr. Taylor said he remained optimistic about Ukraine funding, since both the Democratic and Republican leadership in Congress have expressed support and because the proposals by the far-right flank are almost certain not to pass the House.
Throughout the week, Mr. Biden and other American officials have been intent on just ensuring unity in backing for Ukraine — at the NATO summit and back home. When a Ukrainian activist pressed Jake Sullivan, Mr. Biden’s national security adviser, over the U.S. administration’s reluctance to invite Ukraine to join the alliance immediately, Mr. Sullivan reminded her that the Biden administration had provided “an enormous amount of capacity” to Kyiv.
He then invoked those within U.S. borders. “The American people have sought — in watching and wanting to stand in solidarity with the brave and courageous people of Ukraine — to step up and deliver, and I think the American people do deserve a degree of gratitude from us,” he said.
And in describing the war as a choice between democracy and autocratic governments — a message he has leaned on since the start of his presidency — Mr. Biden sought to convince voters that they should care about a battle on the other side of the globe.
“A choice between a world defined by coercion and exploitation, where might makes right,” Mr. Biden said, “or a world where we recognize that our own success is bound to the success of others.”
