President Biden’s urgent push to replenish Ukraine’s war chest and send aid to Israel is on the brink of collapse in the Senate, where Republicans are prepared on Wednesday to block the funding unless Democrats agree to add strict measures to clamp down on migration at the U.S. border with Mexico.

A classified briefing with administration officials called to shore up support devolved into a partisan screaming match on Tuesday afternoon, with Republicans angrily accusing Democrats of trying to steamroller over their demands for a border crackdown.

The meltdown, which took place on the eve of a critical test vote in the Senate on a $110.5 billion emergency spending bill, not only made it clear that the measure would fail, but severely dimmed the prospects for any bipartisan agreement soon. A vote to block aid would spotlight flagging U.S. resolve at a critical time in Ukraine’s war against President Vladimir V. Putin’s Russia.

“We are about to abandon Ukraine,” Senator Christopher S. Murphy, a Connecticut Democrat who played a leading role in negotiations with Republicans over the past few weeks, said as he left the briefing with cabinet-level officials. “When Vladimir Putin marches into a NATO country, they will rue the day they decided to play politics with the future of Ukraine’s security,” he said of Republicans.