Chuck Schumer, the Democratic Senate leader, had committed to holding a vote later on adding the border security measures demanded by Republicans in a bid to secure the 60 votes needed to get it over its first procedural hurdle.

But the 49-strong Republican minority in the 100-member upper chamber voted en masse against moving forward, pointing to a lack of government action on the estimated 10,000 migrants crossing from Mexico daily.

“Everyone has been very, very clear on this to say we’re standing firm. Now is the moment,” Senator James Lankford, a lead Republican negotiator on immigration and border issues, told Fox Business ahead of the vote.

“We’re completely out of control at the southern border, and it’s time to resolve this.”

Biden has led the global coalition backing Kyiv, but support has been waning among Republicans in Congress, and the administration has warned that it will run out of money for more Ukraine aid in weeks unless lawmakers act.

The president has been under pressure from progressives to reject sweeping conservative demands on immigration – which they say are akin to closing the border – but he vowed in an impassioned televised address he would accept “significant compromise.”

“This cannot wait. Frankly, I think it’s stunning that we’ve gotten to this point in the first place, where Republicans in Congress are willing to give Putin the greatest gift he could hope for,” Biden said.

The Democratic leader was speaking after a video summit with Ukrainian President Volodymyr Zelensky and the leaders of G7 nations to discuss how to shore up western aid for Kyiv.

Zelensky warned the leaders that Moscow was counting on western unity to “collapse” next year and said Russia had ramped up pressure on the front lines of the war.

But the precarious prospects for the aid package had been clear since a classified Ukraine briefing for senators on Tuesday that saw several Republicans walk out, angry that there was no talk of border security.

Zelensky had been due to address the meeting via video link but cancelled at the last minute.

In the Republican-led House, Speaker Mike Johnson, who voted against aid to Kyiv before he took on this job, has made clear he will not agree to sending any more money without “transformative” changes to border policy.

The Louisiana Republican has also declared that any Israel aid needs to be offset with spending cuts, a policy Democrats, the White House and most Senate Republicans oppose.

Centrist Democrat Joe Manchin – often a thorn in the side of the White House – voiced support for the security package – but only because of Schumer’s pledge that amendments on border security could be added later.

“In the greatest country on Earth, we do not have to choose between protecting our homeland and defending our allies,” he said.

The US State Department separately announced a stopgap US$175 million tranche of new aid for Ukraine on Wednesday, including prized HIMARS rockets, shells, missiles and ammunition.