WASHINGTON (AP) — It's one of the most hard-fought provisions in the bipartisan border security package, and one that Democrats had to be persuaded to include: a new authority for the U.S. government to block migrants from entering the country.

Now it's the central line of attack from many Republicans who are opposing the legislation, with some now claiming it would incentivize even more border crossings.

At issue is a provision in the bipartisan package that would grant the Homeland Security secretary emergency authority to prohibit entry for most individuals if an average of more than 4,000 people per day try to enter the country unlawfully over the course of a week. If the number reaches 5,000 or if 8,500 try to enter unlawfully in a single day, use of the authority would be mandatory.

The bill, released by senators on Sunday, would also make it harder to claim asylum at the border and expand detention facilities, among other efforts to reduce the number of migrants.

If the proposal were passed into law, the new authority could be triggered almost immediately, given that border encounters topped 10,000 on some days during December, which was the highest month on record for illegal crossings. President Joe Biden has said he would use the authority to "shut down" the border.

Still, many Republicans say the number should be zero. And some have even created the impression that the bill actually would allow 5,000 additional migrants in a day, or loosen current standards.

The legislation would "further incentivize thousands of illegals to pour in across our borders daily," posted New York Rep. Elise Stefanik, the No. 4 Republican in the House, on X, formerly known as Twitter.

Even before the text was released, former President Donald Trump called the idea of a 5,000 threshold "record-setting stuff" and said supporting the package — which also includes $60 billion in wartime aid for Ukraine — is a "death wish" for Republicans.

The swift and loud opposition from GOP lawmakers who have long called for stricter border measures has frustrated some members of their own party. The backlash suggests the bipartisan bill has little chance of passage, especially in an election year. House Speaker Mike Johnson called it "dead on arrival," and a new hashtag has appeared on the official X account for House Republicans: #killthebill.

A look at what the bill would do, and how some Republicans are trying to stop it:

NEGOTIATORS PUSH BACK

The three main negotiators on the Senate bill — Republican Sen. James Lankford of Oklahoma, Independent Sen. Kyrsten Sinema of Arizona and Democratic Sen. Chris Murphy of Connecticut — have all pushed back on criticism of the bill. They emphasize that it would keep more people out instead of allowing more people to come in — and that migrants would not be able to apply for asylum at all if illegal border crossings reach certain numbers.

The policy is similar to one first used by Trump. Known as Title 42, it justified the quick expulsion of migrants from the country in the name of stopping the spread of COVID-19.

Lankford has repeatedly emphasized that the emergency authority "is not designed to let 5,000 people in, it is designed to close the border and turn 5,000 people around."

After meeting with Republicans in the Capitol Monday evening, Lankford told reporters that people understand it once he explains it, "but it's been said wrong so many times that people immediately just go back to, 'this lets 5,000 people in a day,' which is just factually wrong, but if you say it enough, it just sounds true."

At the same time, Lankford signaled openness to tweaking the bill as Republicans have lined up against it.

Sinema noted that many of those people are now released into the country under current law. "We are giving tools to this administration and future administrations to actually gain control of the border," she said.

Murphy posted on X: "This line of attack (the bill "accepts 5,000 illegal immigrants a day") is just made up bad faith nonsense."

But some Republican critics of the bill said that they wouldn't accept any border security measures unless it shut down the border completely.

Idaho Sen. Jim Risch, who has been a strong supporter of the Ukraine aid, posted on X: "One illegal immigrant. That's one too many. 5,000? Absolutely not."

MOVING THE GOAL POSTS

As Biden's handling of the border became a top political issue for Trump and his party, House Republicans last fall first pushed the idea of pairing the Ukraine aid with border security. Some Republicans who were opposed to the Ukraine money privately speculated that Democrats would never support tougher border enforcement.

But as Democrats reluctantly embraced the idea as a way to pass the Ukraine aid and began serious bipartisan negotiations in the Senate, House Republicans made clear that they were not in the mood for compromise.

Johnson, newly elected after the ouster of former Speaker Kevin McCarthy in October, signaled on a trip to the border at the first of the year that he would only accept a stricter House-passed version of border security legislation, which was a nonstarter with Democrats.

"If it looks like H.R. 2, we'll talk about it," Johnson said of any border legislation that emerges from the Senate.

After the Senate's bill text was released on Sunday, House Republicans and some Senate Republicans immediately said they would oppose it. Many now say legislation isn't needed at all because Biden has the authority to make changes needed — while at the same time pushing their own version of the bill.

"Make no mistake, President Biden's policies are entirely responsible for the crisis at the border — there is no legislative fix for his gross recklessness," said Rep. Carlos Gimenez, R-Fla.

SHUT DOWN THE BORDER?

Biden has said that if the bill passes, it would "give me as president the emergency authority to shut down the border until it could get back under control. If that bill were the law today, I'd shut down the border right now and fix it quickly."

If the authority were used, trade would continue, people who are citizens and legal residents could continue to go back and forth. Migrants could still apply at ports of entry, and once the average of illegal crossings dropped by 75%, the administration would have two weeks to end the use of the emergency authority.

ALL ABOUT THE POLITICS

Regardless of the details, some Republicans have acknowledged that the divisive politics of the issue might be too much to overcome, especially in an election year. And Trump, the front-runner for the GOP presidential nomination, has denounced it, making it tougher for many Republicans to support.

Senate Republican Leader Mitch McConnell, who has backed the compromise, acknowledged the challenge in a meeting with GOP senators two weeks ago. The border issue used to unite us, he said, but now it divides us. He acknowledged it may not have the votes to pass, even though he said he personally supports it.

Oklahoma Sen. Markwayne Mullin, a Republican and former House member, said last week that the perception the bill wasn't strong enough was "already out there" even before the text was released.

"You have a lot of people up for reelection," Mullin said. "And the perception of the American people is that is bad. So it's really hard to get ahead of that."

Rep. Dan Crenshaw, R-Texas, wrote in a post on X Sunday evening that "immigration law is complex and confusing."

"Most members of Congress have not actually studied the problem, even though they voice strong opinions," he said. "So it's easy to spin narratives because people don't know any better."

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Associated Press writers Stephen Groves, Lisa Mascaro and Farnoush Amiri contributed to this report.