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.

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