Bipartisan U.S. Senate talks on a border security deal that some have set as a condition of further aid for Ukraine and Israel have hit a critical point, lawmakers said on Thursday, with a top Senate Republican saying a “plan B” may be needed to provide more aid for U.S. allies.

A small group of senators has spent months trying to iron out an agreement to address the flow of migrants across the U.S.-Mexico border. But the effort has recently encountered growing opposition among Republicans aligned with Donald Trump, the frontrunner for their party’s presidential nomination.

“We’re at a critical moment, and we’ve got to drive hard to get this done. And if we can’t get there, then we’ll go to Plan B,” Senator John Thune, the chamber’s No. 2 Republican, told reporters in the U.S. Capitol.

“For now, at least, there are still attempts being made to try and reach a conclusion that would satisfy a lot of Republicans.”

Senate Republican leader Mitch McConnell and other top party members, including Thune, want a border deal that can win support from most Senate Republicans, in hopes of prompting the Republican-controlled House of Representatives to take up the measure combining border security with aid for Ukraine, Israel and Taiwan.

McConnell has been the most prominent Republican advocate for additional U.S. aid to Ukraine to help it fight off Russia’s invasion, even as a growing number of his party in the House and Senate express skepticism about the value of spending more to help an ally. It was not clear what path McConnell might take on foreign aid if a border deal failed.

But Republicans aligned with Trump have become more voluble in their skepticism since the former president took to social media to warn against any deal that fails to deliver everything Republican need to shut down border crossings.

The White House said there was no reason for bipartisan border talks in the Senate not to continue.

“We need to come together on (a) common sense compromise on border measures and border policy and border resources. And we still are hopeful that that can happen,” White House spokesperson Olivia Dalton said.

Senator Chris Murphy, the Democratic lawmaker in the negotiations, said Republicans will have to decide whether to accept a border deal that has yet to be finalized.

“We have produced the compromise that they (Republicans) asked for, with the chosen negotiator that they appointed, and it is now up to them as to whether they want to accept the agreement,” the Connecticut lawmaker told reporters.

Punchbowl News reported that McConnell told Republicans in a private meeting that the time and political will to pass a bipartisan border deal were running out, and that Republicans should not undermine Trump’s intention to focus his White House campaign on immigration.

A McConnell spokesman said on Thursday that he could not dispute the Punchbowl account.

Senator Thom Tillis, a North Carolina Republican who attended the meeting and has been involved in the negotiations, said it was “patently false” that McConnell had changed course.

ALIGNING WITH TRUMP

More than half of the 49 Senate Republicans have endorsed Trump, and many want the conference to reflect the former president’s positions on major issues, including the border.

“Republicans should pay very close attention to the position that President Trump adopts regarding the border. He’s the only president who’s proven he can bring it under control,” said Republican Senator Bill Hagerty.

But Republican Senator Todd Young said it would be a mistake to give up on the issue now by taking negotiations off the table to “pay fealty to short-term considerations.”

“I don’t believe we should take this off the table, certainly not to clear the way for a clean campaign debate season,” Young told reporters. “Let’s get something consequential done for the American people.”

Some Republicans also maintain that they will not vote for a border agreement that fails to win support from a majority of Republican members.

“I generally don’t support voting on things that divide us, unless there’s a good reason to do so,” Republican Senator John Cornyn told reporters.