WASHINGTON — With time slipping to bolster Ukraine’s defenses, Senate negotiators struggled Wednesday to finalize a bipartisan deal that would pair policy changes at the U.S. southern border with wartime aid for Kyiv as their carefully negotiated compromise ran into strong resistance from House Republicans and Donald Trump.

Senate negotiators have kept a close hold on the details of a bipartisan package on border enforcement and immigration policies that was supposed to unlock Republican support in Congress for aiding Ukraine. But conservatives view the tens of billions of dollars in proposed support with growing skepticism, unmoved by arguments about the larger stakes for global security. NATO Secretary-General Jens Stoltenberg was making the case for the aid on Capitol Hill Wednesday, including at the Heritage Foundation, a power center for Trump’s allies in Washington.

President Joe Biden, who is pushing for a deal alongside Republican and Democratic leaders in the Senate, faces a daunting task in convincing Republicans to defy Trump’s wishes and embrace the deal — especially in the midst of an election year.

Republican leaders, including House Speaker Mike Johnson, had looked to February as a potential deadline to approve another tranche of military aid for Ukraine. But the $110 billion national security package that congressional leaders say is essential to buttressing American allies around the globe, including Ukraine, Israel and Taiwan, has been swept up in the fight over border policies.

Meanwhile, Ukrainian troops are running short of weapons, including air defenses and artillery to defend against Russia’s ongoing attack. The Pentagon reported last week it is out of money for Ukraine and unable to send the ammunition and missiles needed to fend off Russia’s invasion.

Senate Majority Leader Chuck Schumer warned in a morning floor speech that “the survival of Ukraine is on the line.”

“The only way we’ll rise to the occasion is if both sides are serious about finding a bipartisan compromise,” he said, adding, “We have not concluded negotiations so we will keep going to get this done.”

Even if the Senate is able to finish the deal and pass it, resistance is strong in the House, where Trump, the likely Republican presidential nominee, holds significant sway over lawmakers. Trump’s opposition has left Republican leaders increasingly questioning whether the border legislation should be jettisoned from the package in a last-ditch effort to get the Ukraine funding through Congress.

“It’s time for us to move something, hopefully including a border agreement, but we need to get help to Israel and to Ukraine quickly,” said Senate Republican Leader Mitch McConnell.

Senate Republicans are currently divided on the border policy changes. Though many are waiting to study the bill text, it has already been denounced by conservative advocacy groups and Trump as insufficient to clamp down on illegal immigration.

“The perception is already out there,” said Sen. Markwayne Mullin, an Oklahoma Republican. “And the perception of the American people is that this is bad. So it’s really hard to get ahead of that.”

Johnson discussed the idea of splitting up parts of the national security package in a Tuesday meeting with the speakers of the parliaments of Latvia, Estonia, and Lithuania, but did not commit to any course of action, according to a person familiar with the meeting who spoke anonymously about the private discussion.

The speaker has long been skeptical of sending economic assistance to Kyiv, though he has said he wants to halt Russian President Vladimir Putin’s advance in Europe. But a large portion of Johnson’s conference in the House is more firmly against the aid.

Stoltenberg, the longest-serving chief in NATO’s history, pleaded Wednesday for lawmakers to act.

“I met many politicians from both parties. And I saw broad support for Ukraine,” Stoltenberg told CNN Wednesday. “But then, of course, there’s this link to the border issue which I respect is an important and difficult issue, but I believe it’s possible to find a way forward to support Ukraine regardless of how the border issue is handled.”

In a speech Wednesday at the Trump-aligned Heritage Foundation, Stoltenberg warned that Putin’s ambitions don’t just end with Ukraine. He said the Russian president is intent on “reestablishing Russia’s sphere of influence and shaping an alternative world order.”

As Russian forces and drone attacks pummel the region, the Ukrainians will face increasing difficulty defending their cities and populations from incoming assaults.

Yet Republicans also want to cut portions of the package that would not go directly to Ukraine’s defenses. Of the $61 billion in the package for Ukraine, a portion, about $16 billion, would go toward economic, security and operational assistance.

The U.S. economic aid has been keeping the Ukrainian government functioning, paying for public works and employees and the services they provide, but Republicans chafe at the expenditures. They prefer the U.S. focus its spending on military hardware to win the war.

The economic assistance for Ukraine is expected to be trimmed back in the final supplemental package, according to a person familiar with the situation and granted anonymity to discuss it. The person said changes in the amount of humanitarian aid for Gaza, which was stripped from the package by House Republicans, are also being discussed.

Senate Republicans initially insisted on pairing border policy changes with Ukraine aid as part of a strategy to push the package through Congress. But so far, compromising on border policies has only made things more difficult.

Johnson, who has consulted with Trump on border policy in recent weeks, told fellow Republicans in a closed-door meeting on Tuesday morning that one of the Senate border proposal’s key compromises is a “non-starter” in the House. He pointed to a presidential authority to deny migrants the ability to apply for asylum at the border once illegal crossings rise above 4,000 daily, according to those in the meeting. The proposal would require expulsions at the border if the number reached 5,000.

The speaker wants the expulsion authority to be mandatory no matter the number of crossings. While he has said he has not passed final judgement on the bill, he is poised to reject any compromise.

Sen. Kyrsten Sinema, an Arizona independent who has been central to Senate talks, urged lawmakers to keep an open mind to the legislation and said the group was close to releasing text, but was still working through the intricacies of writing immigration law.

She said it was “factually false” for conservatives to claim that the proposal would allow 5,000 migrants to enter the country daily. The expulsion authority is one part of a new system that raises the initial standard to receive asylum protection and quickly processes asylum claims.

Migrants who apply for asylum at ports of entry would be put in a “removal authority program,” in which their asylum case is decided within six months, Sinema said. And migrants who seek asylum in between ports of entry would be put into detention and removed within 10 to 15 days if they fail the initial interviews, known as credible fear screenings.

“It ensures that the government both has the power and must close down the border during times when our system is overwhelmed, and it creates new structures to ensure that folks who do not qualify for asylum cannot enter the country and stay here,” she said. “It is a very robust package.”

Sinema said Johnson’s team is familiar with the details of the bill.

Still, Johnson on Wednesday used his inaugural floor speech since becoming speaker to lay blame on Biden’s handling of the border and rally Republicans to insist on hardline border measures, even though those policies have virtually no chance of passing the Senate.

“If we take a step back, if we consider the current catastrophe at the border, we can all see that our country is at a critical decision,” he said.