WASHINGTON — In a striking turn of events, Senate Republicans threatened Monday to block a major bipartisan package of border security measures and asylum restrictions, just one day after their chief negotiator signed off on it.

GOP senators left a special closed-door meeting in the evening predicting that their party would not provide enough votes to move forward with the package Wednesday, saying they agreed they need more time to discuss changes to the bill in the form of amendments.

“I would anticipate Wednesday the cloture vote does not pass,” Sen. James Lankford, R-Okla., the lead GOP negotiator in the border talks, told reporters after the meeting. “People are saying, ‘Hey, I need a lot more time to be able to go through this.’”

The Republican uneasiness could be devastating to the package, which House Republican leaders have already said is “dead on arrival” in the lower chamber. Proponents hoped that strong bipartisan support in the Senate could force the House’s hand.

Donald Trump demands that Republicans sink the agreement, which they struck with Democrats and is now backed by President Joe Biden, as Trump, the likely 2024 Republican presidential nominee, seeks to wield immigration as a political weapon in the fall election. Trump tore into the bill on social media, calling it “nothing more than a highly sophisticated trap for Republicans to assume the blame on what the Radical Left Democrats have done to our Border, just in time for our most important EVER Election.”

Senate Minority Whip John Thune, R-S.D., told reporters that Republicans worry there “hasn’t been adequate time” to process the bill yet. “I think it’s fair to say everybody thinks that voting Wednesday is voting too soon,” he said.

The 370-page bill, finalized and released Sunday, was crafted with the input of Senate Minority Leader Mitch McConnell, R-Ky., who advocated for it on the Senate floor Monday, warning of crises at the southern border and internationally. In addition to new border provisions, the package includes aid money for Ukraine and Israel. “It’s now time for Congress to take action on supplemental national security legislation that finally meets those challenges head-on,” he said.

But just hours later, when Senate Republicans met behind closed doors and opposition continued to grow, McConnell — one of the staunchest GOP proponents of Ukraine aid in Congress — gave his members the green light to oppose Wednesday’s procedural vote. McConnell told Republicans that if they have reasons to vote against the bill, they could, given that talks on amendments and how to proceed are ongoing, said a source familiar with the meeting.

Democrats were stunned to see Republicans abandon the pact.

“Just gobsmacked. I’ve never seen anything like it,” Sen. Brian Schatz, D-Hawaii, a member of Democratic leadership, said on X. “They literally demanded specific policy, got it, and then killed it.”

Sen. Chris Murphy, D-Conn., the chief Democratic negotiator, called the Republicans’ skittishness about their own agreement “embarrassing.”

“You told us you wanted a bipartisan border fix. You appointed the Republican negotiator. WE GOT A DEAL,” he wrote on X. “Stop the drama — do you want to fix the border or do you want to keep the border chaotic to help Trump? Just decide pls.”

The vote currently scheduled for Wednesday is a procedural vote to move toward debating the legislation. That vote, called a cloture motion, needs 60 votes to pass. If it fails, the bill would be indefinitely on ice until 60 senators agreed to restart floor consideration.

Senate Republicans left Monday’s meeting calling the discussion “robust”; at one point, reporters could hear Sen Ron Johnson, R-Wis., speaking loudly in the room, to which Sen. Joni Ernst, R-Iowa, could be heard saying, “Time out!” Sen Mike Lee, R-Utah, an outspoken opponent of the bill, left the meeting saying the room was “not super bullish” on the immigration package.

Sen John Kennedy, R-La., said: “It was a debate over provisions, substance, process. Even if I understood the bill completely, and we’re going through it line by line, I wouldn’t vote for cloture out of respect for my colleagues, because a lot of them, I sense, are not as far along as we are.”

Within minutes of Sunday’s release of the bill, conservative senators and House Republicans began blasting it. House Speaker Mike Johnson, R-La., and his leadership team issued a joint statement pronouncing it “DEAD” — in all caps — in the House.

Lankford was taken aback at the tenor of the GOP opposition.

“Quite frankly, I was surprised at some folks that said, ‘It’ll take me days and weeks to be able to read through the bill,’ yet within a few minutes they tweeted out their opposition,” he said in an interview earlier Monday.

Sen. Rick Scott, R-Fla., an outspoken McConnell critic who unsuccessfully challenged him for the leadership position 15 months ago, wrote on X: “Again and again, Senate GOP Leadership has shoved bad bills through without questions or input from members. Today, we said enough.”

Sen. Kyrsten Sinema, I-Ariz., a co-author of the package, told reporters Monday that there’s “a lot of misinformation that is still swirling about the legislation” and fretted that election-year politics was driving some of the opposition.

“We raise the asylum standard. We increase detention beds so that single adults who come in and go into detention as they get their asylum interview — and then they are removed from the country if they’re unable to provide proof at the higher level of screening. Family units — we cannot detain them because of Flores — go under supervision, and they get their interview within 90 days. If they are not able to provide proof of the higher standard with the three bars in those 90 days, they are swiftly removed from the country.”

Lankford, addressing whether the experience makes him hesitant to wade into another negotiation in the future, said: “I need a nap. So for me, I’m not interested in jumping into the next big thing.”