The court battle is the latest border policy clash between the Biden administration and Texas Gov. Greg Abbott (R), who says the president’s immigration record has been too lax and has hurt his state by creating incentives for illegal entries. Abbott has led a push by Republican-run states to take on a direct role in immigration enforcement that is playing out amid a presidential race in which border security has emerged as a vulnerability for President Biden after three years of record crossings.

Illegal crossings along the Mexico border have averaged 2 million per year during Biden’s term, the highest levels the U.S. Border Patrol has ever recorded.

Wednesday’s hearing followed a day of legal whiplash in federal court for the Texas law, known as S.B. 4. The Supreme Court briefly allowed the law to take effect, but the U.S. Court of Appeals for the 5th Circuit halted the state from enforcing it late Tuesday.

Fifth Circuit Chief Judge Priscilla Richman asked during the hearing Wednesday how the Texas law would work in practice, listing scenarios that could quickly lead to confusion.

“This is the first time, it seems to me, that a state has claimed that they have the right to remove illegal aliens,” Richman said. “This is not something, a power, that historically has been exercised by states, has it?”

State officials said they would not deport migrants directly but would hand off detainees to federal officials or take them to border crossings with Mexico.

Richman asked what would happen if federal officials, as they have said, refused to carry out an order. She asked what would happen if a foreign national entered the United States via Canada and crossed through several states on their way to Texas. Could they be arrested and deported under Texas’s new law?

Aaron Nielson, the Texas solicitor general, said he wasn’t sure. In some cases, they might be arrested; in others, they might not. Nielson said the Texas law is “uncharted.”

Nielsen said Texas has the right to arrest people for entering the state illegally.

“Texas has decided that we are at the epicenter of this crisis,” he said. “We are on the front line, and we are going to do something about it.”

The law’s fate is yet another flash point in the nation’s polarized debate over immigration, which former president Donald Trump, the likely Republican nominee in the 2024 election, has made a central theme of his campaign against Biden.

Whatever the 5th Circuit decides, the status of the law is likely to go back before the Supreme Court.

The high court’s order Tuesday afternoon set off a fast-moving round of legal maneuvering in the lower court that has kept the law’s status in limbo.

The Supreme Court urged the 5th Circuit to decide quickly whether the law would remain in effect while litigation continues.

The brief order late Tuesday once again blocking the law did not explain the reasoning of the two judges — Richman, a nominee of George W. Bush, and Irma Carrillo Ramirez, a Biden nominee. The dissenting judge — Andrew Oldham, a Trump nominee — said he would have allowed the law to remain in effect before the hearing Wednesday.

“It’s ping-pong,” Efrén C. Olivares, director of strategic litigation and advocacy at the Southern Poverty Law Center, said in a phone interview, describing the back-and-forth rulings.

Olivares said it is unclear how soon the three-judge panel will rule, since a preliminary injunction from a lower court halting the law remains in place.

The law makes it a state crime for migrants to illegally cross the border and gives Texas officials the ability to carry out their own deportations to Mexico.

How they would do so remains unclear. The Mexican government has said it would not accept anyone sent back by Texas and condemned the law as “encouraging the separation of families, discrimination and racial profiling that violate the human rights of the migrant community.”

Mexican President Andrés Manuel López Obrador on Wednesday referred to the Texas law as draconian.

“It disrespects human rights. It’s a completely dehumanizing law. It’s anti-Christian, unjust. It violates precepts and norms of human coexistence,” López Obrador said. “It doesn’t just violate international law but [the teachings of] the Bible. I say this because those who are applying these unjust, inhumane measures go to church. They forget that the Bible talks about treating the foreigner well and, of course, loving your neighbor.”

The Mexican government hasn’t said how it would respond if Texas officials attempted to send migrants back over the border.

“We don’t want to reveal what we could do if in Texas, the governor and these anti-immigrant, anti-Mexican types attempt to carry out deportations,” López Obrador said. “Let me be clear, we will not accept deportations from the Texas government. And we will not just sit around with our arms crossed.”

The Texas law was passed last year as part of Abbott’s push to expand the state’s role in immigration enforcement by busing migrants to Northern cities, mobilizing thousands of National Guard troops and lining the banks of the Rio Grande with barriers and razor wire.

The Supreme Court’s decision drew dissent from the three liberal justices, two of whom said the majority was inviting “further chaos and crisis in immigration enforcement.”

“This law will disrupt sensitive foreign relations, frustrate the protection of individuals fleeing persecution, hamper active federal enforcement efforts, undermine federal agencies’ ability to detect and monitor imminent security threats, and deter noncitizens from reporting abuse or trafficking,” wrote Justice Sonia Sotomayor, joined by Justice Ketanji Brown Jackson.

The brief seesaw implementation of the law Tuesday appeared to illustrate the dissenting justices’ concerns about legal chaos. Texas Republicans celebrated the Supreme Court ruling on social media Tuesday and said S.B. 4 was in full effect, only to be stopped again hours later.

The Texas law carries state criminal penalties of up to six months in jail for migrants who illegally enter from Mexico. Those who reenter illegally after a deportation could face felony charges and a 10-to-20-year prison sentence. Texas lawmakers also empowered state judges to order deportations to Mexico and allowed local law enforcement personnel to carry out those orders. Judges may drop state charges if a migrant agrees to return to Mexico voluntarily.

Luis Miranda, a spokesman for the Department of Homeland Security, said federal immigration agencies do not have the authority to assist Texas with the implementation of the state law. The only deportations that U.S. agents are allowed to conduct must involve federal orders, he said.

“Immigration is within the exclusive purview of the federal government,” Miranda said in a statement.

As Texas fights to implement its crackdown, Iowa’s Republican-dominated state legislature has passed a similar measure, though narrower in scope. Iowa’s bill empowers state authorities to arrest and issue deportation orders to immigrants in Iowa who had been deported or denied entry into the United States. Gov. Kim Reynolds (R) has said she will sign it into law.

U.S. District Judge David A. Ezra temporarily blocked the Texas law last month, saying it was probably unconstitutional and “could open the door to each state passing its own version of immigration laws.” Ezra said the law intruded into federal matters even more than an Arizona immigration law that the Supreme Court partially struck down in 2012.

But the 5th Circuit quickly froze Ezra’s decision without explanation and said the Texas law could be enforced, at least temporarily, unless the Supreme Court weighed in.

Richman said at the hearing Wednesday that Texas’s argument echoed late Supreme Court Justice Antonin Scalia’s dissent in the court’s ruling over Arizona’s 2010 immigration law. The majority struck down parts of it, but Scalia had disagreed, saying states should be able to exclude people who are there unlawfully.

“Yes, your honor, I think we are saying that,” Nielson said. “And I think trespass is a great example, because Texas does arrest people who come across the border and trespass on Texas property.”

“And you don’t need Senate Bill 4 to do that,” the judge interjected. “You’ve got trespassing laws.”

“Correct,” Nielson said. “But if that is lawful, then I don’t see why this extra provision would be unlawful.”

The Supreme Court in 2012 struck down parts of Arizona’s law, then the most sweeping state immigration law in the country, citing the federal government’s broad powers over immigration. But the high court let some provisions stand, including one that allowed police to question a person about their immigration status.

Arizona Republicans in the state legislature passed a measure in recent weeks to make it a crime for migrants to cross the border illegally. Gov. Katie Hobbs (D) vetoed it, saying attacking immigration is “anti business.”

Daniel Tenny, a Justice Department lawyer arguing the case for the Biden administration Wednesday, said the federal government has clear authority over immigration law and is acting to enforce it. He urged the judges to keep Ezra’s ruling in place blocking the law from taking effect. If not, he said the government would appreciate time to prepare to appeal to the Supreme Court.

Ann E. Marimow, Arelis R. Hernández in El Paso and Mary Beth Sheridan in Mexico City contributed to this report.