Amid battle over Senate border bill, here's how it would overhaul the system

A new bipartisan bill in the Senate seeks to address one of the most complex and politically fraught issues facing the United States: what to do about the southern border.

Though the proposal faces increasingly dim prospects of becoming law, its supporters have said that it would make "dramatic" changes to the country's immigration system.

Broadly, the legislation would accomplish much of what the Biden administration has pushed to do for the past three years and attempts to walk a line between expanded enforcement measures -- long a goal of Republicans -- and more narrowly targeted humanitarian relief.

While preserving certain avenues for legal immigration, the legislation would trigger hard-line restrictions at the border like an emergency power to deny many migrants entry if too many people attempt to cross illegally into the U.S. at once.

But as perhaps any compromise would, the proposal has drawn criticism from both ends of the political spectrum.

"This bill is even worse than we expected, and won't come close to ending the border catastrophe," House Speaker Mike Johnson said in a statement on X, formerly known as Twitter.

Johnson, who blames President Joe Biden, had criticized the Senate negotiations even before the text of the deal was released -- including in his first floor speech as speaker. He and other House Republican leaders claim that key provisions are riddled with "loopholes."

Biden, on the other hand, said that he wanted the bill to become law and could use its new authority to "shut down" the border -- raising questions from Democratic critics about what such a closure would look like and what to do with asylum-seekers who hang in the balance.

Migrant advocates said that while the bill speeds up processing, for example, faster paced adjudication of humanitarian claims that would be seen under the proposal could also result in vulnerable people getting sent back into harm's way and the new restrictions on people who cross between ports of entry will likely put many of those seeking asylum in jeopardy.

Migrants would still be allowed to apply for asylum at U.S. ports of entry with a new process for deciding claims more quickly.

"I view this proposal as the opening of an opportunity to get the reform we need," the executive director of the American Immigration Lawyers Association, Ben Johnson, said on Monday. "But to get there, at the end of the day, we're going to need a lot less dangerous and, quite frankly, inaccurate language from politicians around this issue and around this particular proposal."

Immigration lawyers and policy experts told ABC News that the bill, which is set for a key test vote in the Senate on Wednesday, would leave unsolved many of the challenges seen at the border.

That's because more options for migrants to arrive and stay in the U.S. legally, these experts argue, are essential to discouraging illegal crossings and reducing the number of people at the border, who have sometimes overwhelmed the system with asylum claims.

Immigration opponents, by contrast, say expanded enforcement and arrests are needed.

The bill opens some new avenues for immigration, including 250,000 additional visa slots, some of which will be available for families and others for employment in the U.S.

At the same time, it greatly expands emergency authority for rapidly expelling migrants.

American Immigration Council Executive Director Jeremy Robbins called the legislation "a hugely blunt instrument, the way this is drawn up." He pointed to part of the bill that would trigger the ability to implement widespread, fast-track expulsions if more than 5,000 migrants are encountered in a single day.

Republicans have seized on the 5,000 number, saying it still allows too many unauthorized immigrants into the county. But that number is merely a threshold for new asylum restrictions to begin.

Migrants who illegally enter the U.S. below the 5,000-mark would still be subject to deportation if they don't have a valid humanitarian claim.

The union representing Border Patrol agents backs the bill. National Border Patrol Council President Brandon Judd told ABC News that while it does not go far enough in restricting migration, the proposal is better than the status quo because of how it increases enforcement and expands institutional resources, like for processing.

“If they're [migrants are] released, they can abscond,” Judd acknowledged, referring to what detractors call the risk of so-called "catch and release."

“But," Judd said, "at least there's some very serious consequences if they do that. And because of those things, I do believe that this will drop illegal immigration. I don’t ever think we will hit the 5,000-mark under this.”

With more resources for migrant processing, Judd said more agents will be able to return to the front line to perform their typical duties. Previously, patrol agents were pulled into processing centers to document unauthorized migrants and help ensure they have court dates for removal proceedings.

Questions remain about how the Mexican government would respond to the new expulsion authority given the country's reluctance to accept migrants in the past.

"There are a lot of assumptions in this about what the Mexican government will do that are not borne out by factuality," Robbins said.

Immigrant advocates worry that large groups of migrants will be stuck in dangerous Mexican border towns and will be vulnerable to exploitation.

The U.S. State Department maintains travel warnings for most of the Mexican side of the southern border given the high rates of violent crime.

When asylum-seekers were forced back into Mexico under the Trump administration's "Remain in Mexico" policy, human rights groups documented volumes of violent crimes against those waiting.

"And unfortunately, as we saw back then, these encampments ... they'll be unsafe and unsanitary," American Immigration Lawyers Association Senior Director for Government Relations Greg Chen said. "Cartels will likely prey upon these individuals. And so we'll have an increase of crime and violence due to that. It's also not clear that it's going to be an effective deterrent."