The United States Has Been a Bulwark for Ukraine. What Happens if Support Collapses?

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

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

The turnaround has surprised the White House. Even if the Senate manages to advance military aid, there are still plenty of reasons to doubt that the money will come through, including deep opposition among Republicans in the House and former President Donald J. Trump’s push for a more isolationist stance.

President Biden’s aides insist they are not yet scrambling for other options.

“We’re not focused on Plan B,” Jake Sullivan, the president’s national security adviser, said in Brussels on Wednesday after a NATO meeting with his counterparts. “We’re focused on plan A,” which he said meant passing a bipartisan aid package that will enable Ukraine to “defend effectively and to take back territory that Russia currently occupies.”

But behind the scenes there is a lot of discussion, in Washington and Europe, about other options, including seizing more than $300 billion in Russian central bank assets that are stashed in Western nations — a process that is turning out to be a lot more complicated than it first seemed.