Eagle Pass, Texas — A federal judge in Austin on Thursday blocked Texas state officials from implementing a sweeping immigration law that would have allowed them to arrest, jail and prosecute migrants who cross into the U.S. illegally.

The Justice Department and the American Civil Liberties Union had challenged the law known as SB4, which was set to take effect on Tuesday, March 5. 

In a 114-page order, Judge David Ezra of the U.S. District Court for the Western District of Texas granted the challengers' request for a preliminary injunction preventing Texas officials from enforcing the state law.

Ezra wrote that the Constitution and Supreme Court precedent make clear that states cannot enforce immigration measures without federal approval, and that the Texas law conflicts with federal law. He rejected an argument from Texas that the state's authority to repel an "invasion" allows it to enforce SB4, writing that "surges in immigration do not constitute an 'invasion' within the meaning of the Constitution." Lastly, he said that allowing Texas to enforce the law would mean states could override federal statutes, a discredited constitutional theory known as nullification.

"[T]o allow Texas to permanently supersede federal directives on the basis of an invasion would amount to nullification of federal law and authority — a notion that is antithetical to the Constitution and has been unequivocally rejected by federal courts since the Civil War," Ezra wrote.

In a statement, Texas Gov. Greg Abbott vowed to "immediately appeal this decision, and we will not back down in our fight to protect our state — and our nation — from President Biden's border crisis." Attorney General Ken Paxton soon filed notice with the court that the state was appealing the ruling.

The appeal will come before the U.S. Court of Appeals for the 5th Circuit, which could pause the district judge's decision while it reviews the case. Either party could ultimately ask the Supreme Court to review the appeals court's action. 

Abbott noted that "[e]ven from the bench, this District Judge acknowledged that this case will ultimately be decided by the U.S. Supreme Court." The high court could agree to hear the case, or let the lower court's ruling stand as the final word on the issue.

What SB4 would do

Crossing into the U.S. outside of a port of entry is already illegal under federal law, but the Texas law would've criminalized the action at the state level. It would allow state law enforcement officials to stop and detain anyone suspected of entering Texas unlawfully and empower state judges to issue de facto deportation orders.

While Texas troopers have already been arresting some migrant adults who cross the U.S.-Mexico border on state trespassing charges, that effort has required the consent of private property owners. SB4 would not.

Under the law, crossing into Texas illegally from Mexico would be treated as a misdemeanor crime, punishable by up to 180 days in jail and a fine of up to $2,000. Illegal reentry into Texas would be a felony offense, punishable with up to 2, 10 or 20 years in jail, depending on whether the migrant in question had been previously deported or convicted of certain crimes.

SB4 includes a provision that bars state officials from arresting migrants in certain locations, including schools, places of worship and health care facilities.

The law would also allow Texas magistrates to order migrants suspected of committing the new illegal entry or reentry crimes to return to Mexico as an alternative to continuing their prosecution. Those found to violate those orders could be charged with a second-degree felony.

Texas Gov. Greg Abbott, a Republican, signed SB4 into law, arguing it's needed to deter illegal border crossings. More than 2 million migrants were apprehended by U.S. Border Patrol agents along the southern border in both fiscal years 2022 and 2023, the highest levels on record.

But the Biden administration said the state law interferes with federal immigration enforcement. Other critics of the measure have also said it could lead to racial profiling and overwhelm county jails and public safety resources.

Camilo Montoya-Galvez

Camilo Montoya-Galvez is the immigration reporter at CBS News. Based in Washington, he covers immigration policy and politics.

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