Ukraine Aid Falters in Senate as Republicans Insist on Border Restrictions

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.

Republican senators leaving the briefing said they were outraged that administration officials had refused to answer their questions about border security, which they said was a prerequisite for any plan to send emergency help to American allies.

“They want tens of billions of dollars to help our friends and allies overseas, but they’re not willing to do what’s necessary to prevent a potential crisis at the border,” said Senator John Cornyn, Republican of Texas. “The Biden administration just does not seem to care.”

Even Republicans who have argued vociferously for helping Ukraine amid waning support in their party for doing so said there was no appetite for a bill to do so unless it also clamped down on immigration.

“Count me in on Ukraine; if we do not help Ukraine and Putin gets away with this, you’ll be in a war with NATO — I got all that,” said Senator Lindsey Graham, Republican of South Carolina. “You will get a robust Republican vote for Ukraine, Israel, Taiwan if you have real border security to deal with what I think is one of the biggest national security problems I’ve seen since I’ve been up here.”

Mr. Biden called the prospect of allowing aid to lapse “just wrong” on Tuesday night.

“The failure to support Ukraine is just absolutely crazy,” he told reporters. “It’s against U.S. interests.”

“We’re going to get that in,” he vowed.

The planned vote, coming just days after the White House warned that the United States would soon run out of money to send weapons to Ukraine, comes at perhaps the most uncertain moment for the beleaguered nation since the first chaotic months of the war.

Ukraine urgently needs more ammunition and other weapons to try to turn the tide on the battlefield. The country’s counteroffensive against entrenched Russian forces in southern Ukraine has so far failed to meet its objectives, and Moscow’s forces have been going on the offensive in the east.

Senators had expected that President Volodymyr Zelensky of Ukraine would join their briefing via secure video to make a personal appeal for more aid. But Mr. Zelensky canceled at the last minute, leaving the pitch to Biden administration officials: Secretary of State Antony J. Blinken; Defense Secretary Lloyd J. Austin III; Gen. Charles Q. Brown, the chairman of the Joint Chiefs of Staff; and leaders from the Office of the Director of National Intelligence and the U.S. Agency for International Development.

Republicans complained that the roster of speakers included no border officials, accusing Senator Chuck Schumer, Democrat of New York and the majority leader, of refusing to engage on the issue.

“He doesn’t even want to have a negotiation about it. He wouldn’t bring anybody in here to talk about it,” Senator Kevin Cramer, Republican of North Dakota, told reporters heatedly. “Clearly the military people in the room don’t want to talk about it.”

Mr. Cramer confronted General Brown on the matter during the briefing, accusing him of never having been to the border, according to a congressional aide familiar with what transpired, who described the confidential meeting on the condition of anonymity.

Mr. Schumer told reporters that Republicans had “hijacked” the briefing, using it to grandstand about measures to clamp down on border crossings that Democrats have rejected.

“They are feeling a little bit beleaguered because they’re stuck,” he said. “They’re in a box. They don’t know what to do. But we’re going to keep pursuing this.”

Earlier on Tuesday, Mr. Schumer tried to rally G.O.P. support to move forward with the bill by offering them a vote on a border proposal of their choosing. But Republicans said that was not enough.

“I hope all of our members vote no,” Mr. McConnell told reporters. He has been the most outspoken Republican in favor of continuing to arm Ukraine, but he said his party’s opposition to the bill was “to make the point, hopefully for the final time, that we insist on meaningful changes to the border.”

G.O.P. senators have sought to buttress their demands by pointing to a surge in recent border crossings, and a hardening of sentiments in the Republican-led House, where Speaker Mike Johnson has been adamant that a spending measure that does not incorporate major border measures stands no chance of passing.

“Rather than engaging with congressional Republicans to discuss logical reforms, the Biden administration has ignored reality, choosing instead to engage in political posturing,” Mr. Johnson, who has repeatedly voted against Ukraine aid, wrote in a letter on Tuesday to Shalanda D. Young, the director of the White House Office of Management and Budget. He was responding to a letter Ms. Young sent congressional leaders on Monday warning that the U.S. coffers for Ukraine were about to run dry.

“Supplemental Ukraine funding is dependent upon enactment of transformative change to our nation’s border security laws,” Mr. Johnson said, adding that the administration also must provide lawmakers with specifics about how the assistance to Ukraine was being used.

Under pressure from the House, Republicans have twice refused to include military aid for Ukraine in stopgap spending bills to keep the government funded this autumn. That has raised alarms among Ukrainian officials, who swarmed Washington on Tuesday in an attempt to get lawmakers to put aside their political differences.

In remarks at the U.S. Institute of Peace in Washington, Andriy Yermak, the head of the presidential office of Ukraine, said on Tuesday that if Congress fails to approve military assistance for Ukraine swiftly, there is a “very high possibility” that Ukraine will lose the war.

It would be “impossible to continue to liberate, and give the big risk to lose this war,” Mr. Yermak said, addressing the audience in English.

Some Pentagon officials have pushed back against the White House’s claims that military assistance to Ukraine is about to run out, saying that they expected to make the remaining $4.8 billion in aid authority last through the winter.

But Ukrainian officials insist that without an influx of more, they are doomed at best to a stalemate. As weapons shipments have slowed, Ukrainian troops have struggled to advance on the battlefield — and are facing recruitment and training challenges as the loss of soldiers mounts.

Mr. Putin has made it clear he is investing in a long war: Nearly a third of the country’s spending next year, roughly $109 billion, will be devoted to “national defense,” according to a budget he signed into law last week.

Andrew E. Kramer and Eric Schmitt contributed reporting.