The United States has been a bulwark for Ukraine. What happens if support disappears?

A year ago, when Washington and much of Europe were still awash with optimism that Ukraine was on the verge of expelling Russia from its territory, it seemed unthinkable that the United States would turn its back on the victim of Vladimir V. Putin's aggression would turn.

Even as Senate Democrats try to salvage an aid package for Ukraine, that possibility remains real. And the political moment seems a far cry from 14 months ago, when President Volodymyr Zelensky of Ukraine stood before a joint session of Congress, wearing his signature pale green sweater, and basked in a minutes-long standing ovation.

The turnaround took the White House by surprise. Even if the Senate succeeds in advancing military aid, there are still plenty of reasons to doubt that the money will materialize, including deep opposition among Republicans in the House of Representatives and former President Donald J. Trump's push for a more isolationist attitude.

President Biden's aides insist they are not yet looking for other options.

“We are not focused on Plan B,” Jake Sullivan, the president's national security adviser, said Wednesday in Brussels after a NATO meeting with his counterparts. “We are focused on Plan A,” which he said involved passing a bipartisan aid package that will allow Ukraine to “effectively defend and regain territory that Russia currently occupies.”

But behind the scenes, there is much discussion in Washington and Europe about other options, including seizing more than $300 billion of Russia's central bank assets stored in Western countries – a process that is proving to be a lot more complicated . than it first seemed.

Still, U.S. officials admit there is nothing on the horizon that could match the power of a new $60 billion Congress that would buy beefed up air defenses, more tanks and missiles and a huge influx of munitions.

And, they add, the symbolism of America's withdrawal now could be profound.

European officials who have feared the possibility of Trump being re-elected and making good on his promise to withdraw from NATO are beginning to wonder, at least privately, about the reliability of the United States, regardless of who is president.

If Republicans are willing to abide by Mr. Trump's demand to vote against continuing aid to Ukraine, a senior European diplomat asked in Berlin on Wednesday, why should they rely on Mr. Biden's assurances that the United States would defend 'every inch' of aid? NATO territory? Even some of Trump's former national security aides — with whom he parted ways long ago — have begun to say that the failure to finance Ukraine would amount to a huge strategic victory for Putin.

“The United States has a clear choice: arm Ukrainians with the weapons they need to defend themselves, or cut off aid and abandon democratic Ukraine in its struggle for national survival against Putin's aggression,” he said. HR McMaster, who served as second minister for a year. of Trump's four national security advisers, said Monday. He noted that as Congress debated, “abandoning Kiev would be a gift to the Moscow-Tehran-Beijing-Pyongyang axis of aggressors. Allies and partners would lose confidence in America if these aggressors are encouraged.”

Strangely, Congress's threat to derail aid comes just as Europe has pledged $54 billion to rebuild the country over the next four years, and countries from Norway to Germany are providing new arms aid. “It is remarkable how quickly Europe is moving towards a new and substantive multi-year support program for Ukraine,” said Christoph Trebesch, who leads production of the Ukraine Support Tracker at the Kiel Institute for the World Economy in northern Germany. “For the first time, the US is now behind by a large margin” compared to European aid, he said.

“This is not charity; it is in our own security interests,” Jens Stoltenberg, NATO's secretary general, said at the alliance's headquarters on Wednesday, appearing at a news conference with Mr. Sullivan. A Russian victory, he added, “is important for European security and it is important for American security.”

But this argument, that the West must push back Russia in Ukraine or face the possibility of fighting Russia on NATO territory, appears to be losing its effectiveness in Congress. And some Republican members of Congress still accuse Europe of not doing its part, even as the latest financial commitments change the equation.

But none of these arguments, say officials in the US and Europe, can overcome the reality: If the United States pulls the plug on its financial support for the war, many of the day-to-day military needs will disappear – starting with air defense against the almost daily barrage of missiles, drones and other weaponry targeting urban centers and critical infrastructure such as the power grid. And if the country's economy collapses, it will end a two-year effort to save a fledgling but seriously flawed democracy.

Anti-aid Republicans do not directly challenge that logic, though many insist that pouring billions into a country with a deep history of corruption invites abuse. Instead, their main argument is that the money should be spent at home, on the southern border, and not in Ukraine's border area with Russia. The most vocal opponents, including Reps. Marjorie Taylor Greene of Georgia and Matt Gaetz of Florida, argue that aid to Ukraine “puts America last.”

For months, the White House viewed them as a fringe group. But polls show that the percentage of Republican voters who think the United States has spent too much on defending the country is soaring. And now many Republicans have become resistant, aligning their own views with the long-held position of Mr. Trump, who said in the 2016 campaign that he did not want to defend Ukraine. Eight years later, he insists – without giving any details – that he would end the war within 24 hours.

Now the opposition has gained such a foothold that even Senate Republican leader Mitch McConnell of Kentucky, who has stated time and time again that financing the war in Ukraine was one of his top priorities, appeared to be backing down.

Meanwhile, Mr. Biden's aides are trying to figure out how to pay for guns if Congress remains paralyzed. The plan to seize Russian assets has complications. It is not clear whether the reserves can be used to pay for air defense and artillery. Even that, administration officials say, could require congressional action — though there are likely more votes in the House of Representatives and the Senate for spending Russian money than for spending that of the United States.

There is also talk of carrying out complex arms swaps, similar to what Japan and South Korea have done, where they supplied their artillery shells to the United States, freeing Washington to give more to Ukraine. (Both countries have said they cannot export directly to a war zone.) Or perhaps European countries should pay for American weapons and ship them to Ukraine.

But Europe clearly does not have the capacity to supply much more ammunition itself. During the thirty years of increasingly uneasy peace with Russia, Europe has dismantled much of its productive capacity. Ursula von der Leyen, the president of the European Commission, said in a recent speech that “we will have delivered more than half a million artillery shells next month” and “more than a million by the end of the year,” but she acknowledged that “this is definitely not enough.”

Europe also has little to contribute to the production of drones. And Germany remains unwilling to hand over its most powerful air-launched cruise missile, the Taurus, for fear it will be deployed deep into Russian territory. Germany's role will undoubtedly take center stage during a meeting between Chancellor Olaf Scholz and Mr Biden at the White House on Friday.

For his part, Mr. Sullivan insists that if the government sticks to its strategy, it will prevail. “Walking away from Ukraine at this time would be fundamentally wrong for the fundamental national security of the United States and also for our NATO allies,” he said on Wednesday. “And we think we'll continue to win that argument.”

Steven Erlanger contributed reporting from Oslo.