WASHINGTON (AP) — House Speaker Mike Johnson told fellow Republicans on Tuesday that sweeping changes to U.S. border policy would be their "hill to die on" in negotiations over President Joe Biden's nearly $106 billion package for the wars in Ukraine and Israel and other security needs.

Johnson delivered the hard-line message Tuesday morning ahead of classified briefings the Biden administration organized to underscore how desperately the aid is needed. Ukrainian President Volodymyr Zelenskyy was scheduled to address the senators via video but had to cancel his appearance, Senate Majority Leader Chuck Schumer said.

Biden is pushing a reluctant Congress to approve the military, economic and humanitarian aid package, but the injection of border security into the negotiations has made progress difficult.

"The battle is for the border," Johnson said at a news conference. "We do that first as a top priority, and we'll take care of these other obligations."

Moments earlier, Johnson told GOP lawmakers in a closed-door meeting that their "hill to die on" in the negotiations was border policy, according to a Republican in the meeting. Conservatives are pressing for the provisions in H.R. 2, a bill they passed in May that would restart construction of walls along the southern border and make it drastically more difficult for migrants to claim asylum in the U.S.

Johnson reiterated his stance in a letter to the White House on Tuesday, one day after officials warned that the U.S. will run out of funding to send weapons and assistance to Ukraine by the end of the year, threatening its ability to fight Russia's invasion.

In the Senate, Schumer was pushing toward a test vote Wednesday on the emergency funding for Ukraine, Israel, and other national security needs, but without the border provisions Republicans are demanding. He described the package as crucial to ensuring the future of Ukraine and democracy.

"This is a turning point in Western civilization," Schumer told reporters at a news conference.

Schumer added that Johnson told him in a private meeting that he could not pass the supplemental package through the House without H.R. 2 attached.

The GOP's demands could imperil any legislation that emerges from the Senate, where a bipartisan group is trying to find agreement on a pared-down set of border policy proposals. Republicans in those negotiations have acknowledged they are not insisting on the broad policies included in the House's legislation, creating a schism between the two chambers.

Sen. James Lankford, R-Okla., said it was "not rational" to expect the closely divided Senate to pass a bill that didn't gain a single Democratic vote in the House.

"You can't make law like that," Lankford said. "We have to make law."

So far, the Senate negotiations have centered on a proposal to raise the initial threshold for migrants to enter the asylum system, as well as limiting the executive branch's ability to admit migrants through humanitarian parole.

Democrats took a step back from the talks earlier this week, saying that Republicans were unwilling to compromise. Republican senators are making a counter-offer, but still say they will block the funding package if it does not include border security policy they can agree on.

Before the test vote, Schumer made Republicans what he called a "golden offer": agree to move ahead with the aid package and they could offer any amendment they want to add border security to the package.

But McConnell, the Republican leader, showed no interest and is encouraging all Republican senators to vote against advancing the Biden aid package to show they are "serious" in demanding border changes.

"Now is the time to pay attention to our own border in addition to these other important international concerns," McConnell said.

House lawmakers were also set to hear from national security adviser Jake Sullivan about the urgency of providing assistance. Republicans in the House remain deeply skeptical of sending more wartime funding to Ukraine, and some have said they won't support it even if it is paired with hard-line border policy.

The White House has declined to discuss publicly the details of the border negotiations and urged lawmakers to pass Biden's emergency funding request expeditiously.

"I think that the president has been very, very clear and senior administration officials will be very clear to every single member of the House and Senate today about what the stakes are in Ukraine at this moment," Olivia Dalton, the White House principal deputy press secretary, said Tuesday on Air Force One while Biden traveled to Boston for campaign fundraisers.

Johnson, a hard-line conservative, voted against security assistance for Ukraine in September, but since becoming speaker has been more receptive to funding the country's military, warning that Russian President Vladimir Putin cannot be allowed to prevail.

Still, Johnson said he wanted more information from the White House on the strategy for exiting the conflict.

"What is the objective? What is the endgame in Ukraine? How are we going to have proper oversight of the funds?" the speaker said.

The charged dynamic has lawmakers deeply worried that Congress could fail to pass the funding by the end of the year.

"The world needs to be very concerned about what's happening here," said Sen. Chris Murphy, D-Conn., on Monday night. "Republicans have decided to hold Ukraine funding hostage to a domestic political priority that is amongst the hardest in American politics to solve."

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Associated Press writers Kevin Freking and Seung Min Kim contributed reporting.