A new Texas law known as SB4 that gives state officials permission to detain and jail migrants suspected of crossing the U.S. southern border without authorization took effect Monday after the Supreme Court declined to act.

The justices did not address an emergency request from the Justice Department to pause the law before 5 p.m. ET, when an earlier administrative stay expired. The move means Texas can being enforcing the controversial law, one of Gov. Greg Abbott’s signature immigration policies, although the court could still issue an order pausing it as a legal challenge plays out.

Passed by the Texas legislature last year, SB4 criminalizes unauthorized migration at the state level, making the act of entering the U.S. outside of a port of entry — already a federal offense — into a state crime. It also creates a felony charge for illegal reentry at the state level.

At the request of the Biden administration, a federal judge last month blocked SB4, finding that the state measure is at odds with federal immigration laws. That ruling was then suspended by the U.S. Court of Appeals for the 5th Circuit, which is considering the measure’s legality. Justice Samuel Alito subsequently paused the appeals court order until Monday, maintaining the status quo while the court considered the Justice Department’s request for emergency relief.

SB4 empowers Texas law enforcement officials, at the state and local levels, to stop, jail and prosecute migrants on illegal entry and reentry charges. It also allows Texas judges to order migrants to return to Mexico as an alternative to continuing their prosecution, effectively creating a de facto state deportation system.

The Justice Department has said SB4 conflicts with federal law and the Constitution, noting that immigration enforcement, including arrests and deportations, have long been a federal responsibility. It has also argued the measure harms relations with the Mexican government, which has denounced SB4 as “anti-immigrant” and vowed to reject migrants returned by the state of Texas.

Abbott, who has positioned himself as the leading state critic of President Biden’s border policies, has portrayed SB4 as a necessary measure to discourage migrants from crossing the Rio Grande, arguing the federal government has not done enough to deter illegal immigration.

Over the past three years, Texas has mounted the most aggressive state effort yet to challenge the federal government’s power over immigration policy, busing tens of thousands of migrants to major, Democratic-led cities, assembling razor wire and buoys along stretches of the border to deter migrant crossings and filing multiple lawsuits against federal immigration programs. 

Camilo Montoya-Galvez

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