WASHINGTON (AP) — Senate negotiators were laboring Monday to win wide support for a deal on border policy as they entered what could be a pivotal stretch for a painstakingly negotiated compromise that could open the door for Republican votes to replenish U.S. wartime aid for Ukraine.

A core group of negotiators has worked in private for nearly two months over changes to U.S. border and immigration policy and hoped to unveil the legislation later this week.

But the bipartisan group is treading on one of the most explosive issues in American politics, and the legislation faces heavy skepticism from the wings of both political parties, including Donald Trump, the Republican presidential front-runner, and House Speaker Mike Johnson, an arch-conservative who has shown little willingness to compromise on the issue.

With the House on recess, the Senate has an opportunity this week to gain momentum for the initiative. But Republican Senate leaders said they did not expect to vote on the bill this week as negotiations turned to funding the policy changes.

Republican senators want a robust showing of GOP support for the bill to put pressure on Johnson to take it up. Senate Republican Leader Mitch McConnell’s ability to marshal his conference for the border measures could ultimately decide the fate of one of his key priorities: bolstering U.S. support for democratic allies around the world.

“The entire world understands what’s at stake here in the Senate in the coming weeks,” McConnell said in a floor speech.

It’s a goal shared with President Joe Biden, who is requesting $110 billion from Congress to provide wartime aid for Ukraine and Israel, support allies in the Asian-Pacific and overhaul the U.S. immigration system. However, the emergency funding package has been delayed months in Congress after Republicans insisted that border policy changes also be included in the package.

“President Biden has also made clear he’s willing to work with Republicans in a big way — in a big way — on border security,” Senate Majority Leader Chuck Schumer said. “All of us on the Democratic side are serious about getting something done.”

He also cautioned, “It’s certainly not a done deal yet.”

It remains to be seen whether the border policy changes, which have been negotiated in private with top White House officials, will be enough to satisfy most Republican senators. Democrats have questions of their own, including whether the border policies will harm migrants seeking asylum in the U.S., and some in the party are pushing to condition military aid for Israel on humanitarian standards.

A small group of three senators has tried to craft a package that gains support from centrists of both parties.

“You get as many votes as you can on both sides,” said Sen. Kyrsten Sinema, the Arizona independent who has been central to the negotiations. “But yeah, not everyone’s going to be happy.”

The proposal would toughen the asylum process with a goal of cutting the number of migrants who come to the southern U.S. border to make an asylum claim. The group has mostly reached agreements on policy changes, but on Monday was working with Senate appropriators to determine funding levels for the programs.

“We can’t create new funding and new programming and then not fund it,” said Connecticut Sen. Chris Murphy, the lead Democratic negotiator. “So if you’re committed to the policy change, you have to be equally as committed to funding the policy change.”

Biden had already requested $14 billion in the national security package to bolster the immigration system, including sending financial aid to local governments that have absorbed the historic number of people migrating to the U.S. But the Senate proposal also calls for the expansion of a Biden administration program that tracks families seeking asylum with electronic surveillance like ankle bracelet monitors until they are given an initial interview to determine if they are likely fleeing persecution in their home country, according to two people familiar with the talks who spoke anonymously to discuss the private negotiations.

“The overall policy issues we’re pretty well established on,” said Oklahoma Sen. James Lankford, the lead Republican negotiator. He added that negotiators were still reviewing legislative text to make sure it didn’t have unintended consequences.

The Senate proposal would also give federal authorities the ability to expel migrants at the border if the number of encounters reached a certain number. The talks had also floundered on Republican demands to restrict the executive branch’s ability to allow migrants into the country through exceptions to immigration law called humanitarian parole, but negotiators had discussed compromises, including placing limits on how parole can be used.

The policy could still trip up the compromise if many Republicans are displeased that the changes don’t go far enough.

“Parole is the stickler,” Sen. John Cornyn, R-Texas, said after a weekly leadership meeting. “If you leave open the ability to just release people on parole, it creates a backdoor, which doesn’t solve any problem.”

But Democrats are trying to protect a policy that has historically been used to provide urgent relief to people fleeing war and unrest around the world, including from Central America, Ukraine, Afghanistan and Vietnam. The Biden administration has aggressively used parole authority, including by allowing up to 30,000 people a month from Cuba, Haiti, Nicaragua and Venezuela into the country if they have a financial sponsor and a plane ticket.

“I am skeptical of any demolition of the president’s power in this area,” said Sen. Dick Durbin, a Democrat of Illinois, who added that he was waiting to see bill details before passing final judgement.

Durbin also said he was worried about the signs of resistance coming from the Republican-controlled House. Johnson, the speaker, has repeatedly called for Republicans to insist on a conservative plan intended to choke off asylum claims. He has also charged that the problems at the border have been caused by Biden’s policies and could be changed through executive actions.

Still, Congress has not made significant changes to immigration and border law in decades and the politics around it have only become more deeply entrenched.

Democrats have bemoaned the Republican decision to tie Ukraine aid with border policy changes. Schumer, D-N.Y., sought to underscore the urgency of the situation, saying that Ukraine is running low on ammunition.

“The future of the war in Ukraine hangs in the balance,” Schumer said. “The security of our Western democracy hangs in the balance.”