MEXICO CITY — After years of bowing to U.S. pressure to help slow migration, Mexico is drawing the line at a Texas law that would give the state a dramatically bigger role in border enforcement, including the right to deport undocumented migrants.

President Andrés Manuel López Obrador has denounced the measure as “draconian.” He says his government will reject any attempt by Texas officials to send migrants back to Mexico.

In an interview with The Washington Post, Foreign Minister Alicia Bárcena said the government would put “increased vigilance and controls” at border crossings to prevent such removals if the law goes into effect. That raises the specter of standoffs between Mexican and Texas state or local agents over attempted deportations.

“We are not going to accept any return, either of Mexicans or non-Mexicans, from local, state or county authorities in Texas,” she said Wednesday night.

The Biden administration and Texas officials are in a showdown over a Texas state law known as S.B.4, which took effect briefly on Tuesday before a U.S. federal appeals court blocked it.

The Texas government argues that the record number of migrants reaching the border has become an “invasion” that requires the state to take unprecedented action, including arresting and deporting people who are in the country illegally.

The Biden administration says that immigration is a federal responsibility — and that allowing states to craft their own policies would provoke chaos.

If the law is upheld, it could set off the biggest political crisis on the border since 2019, when President Donald Trump threatened to impose tariffs if Mexico didn’t choke off migration. But unlike then, the Mexican government is on the same page as the White House, saying migration agreements can only be forged by national governments — not states.

In Mexico, migration isn’t nearly as contentious as it is in the United States. Yet the Texas law has touched a nerve.

Mexico’s political parties have closed ranks around the government, with leading candidates in the June 2 presidential election condemning the Texas law. “We have to stiffen our leg,” said Xóchitl Gálvez, the top opposition candidate, using a soccer expression meaning to hold one’s ground. “Because the way they’re treating migrants is unacceptable.”

Unclear how Texas would deport migrants

Mexican authorities are concerned that accepting deportees from Texas could lead to a legal free-for-all in which multiple U.S. states could set up their own immigration protocols. “It would create an important precedent,” Bárcena warned, “which I believe would not be good for the U.S. government, either.”

Mexico also worries that the law will increase discrimination against the large Mexican community in Texas. Even people with permission to be in the United States could be detained or arrested by local authorities unschooled in immigration law, migrant advocates say.

S.B. 4 was approved by the Texas legislature and signed by Republican Gov. Greg Abbott last year, but was quickly stalled by legal challenges. It would make it a crime for people to enter Texas from Mexico without authorization, punishable by a six-month jail sentence. Those who crossed the border again after being deported or denied admission could face 10 to 20 years in prison.

The law would empower Texas judges to order deportations and local law enforcement agencies to carry them out.

Texas authorities have acknowledged that they’re not sure how the process would work. They’ve said they might hand detainees over to the federal government for removal. But the Department of Homeland Security has said its agents need approval from federal immigration authorities to deport migrants.

Authorities could also simply drop migrants at international border crossings and tell them to walk into Mexico. Bárcena said the Mexican government has instructed the 11 Mexican consulates in Texas and the national immigration agency “not to accept under any circumstances any kind of returns by local or county authorities” in Texas.

“There are some border crossings at which, of course, we’d put increased vigilance and controls,” she said. “Because we can’t allow this to happen.”

Tonatiuh Guillén, who served as head of Mexico’s immigration agency in 2018 and 2019, said people deported by Texas might just blend into the stream of individuals heading south over the border. But if the government set up filters to stop those ordered removed by Texas, he said, there could be “open tension in the border relationship,” with Texas pushing migrants one way, and Mexico pushing back.

Such filters could drastically slow the movement of the tens of thousands of people who cross the border legally each day to work, study, shop or visit relatives. “I don’t see it as sustainable either on the Texas or Mexican side,” Guillén said.

Mexico has become a key partner in U.S. efforts to contain the flow of migrants bound for the border. In 2018, the incoming López Obrador administration, facing pressure from Trump, agreed to allow U.S. asylum applicants to wait in Mexico while their claims were processed. More than 60,000 asylum seekers were returned under the “Remain in Mexico” program during the Trump administration.

Trump threatened in early 2019 to slap stiff tariffs on Mexican exports if the country didn’t reduce the number of migrants headed for the U.S. border. López Obrador responded with a crackdown involving thousands of Mexican troops.

Cooperation has continued under President Joe Biden. Mexico agreed last year to receive deportees from four countries — Nicaragua, Haiti, Venezuela and Cuba — who had entered the United States illegally.

Mexico says it has accepted such measures for humanitarian reasons, and to help its neighbor. In return, analysts say, the Biden and Trump administrations have muted their criticism of López Obrador on non-immigration matters — such as democratic governance. The president has weakened some of the institutions that have underpinned Mexico’s 21st-century transition to democracy, calling them expensive and biased toward the opposition.

The Biden administration says Mexico has played a crucial role in reducing the number of migrants reaching the U.S. border in the past few months. The country has stepped up detentions of migrants traveling on buses and caught hopping trains, and organized deportation flights to Venezuela, Cuba and Central America. But Mexican officials say privately that the government’s resources are strained. And migration is expected to pick up as temperatures rise in coming weeks.

López Obrador said Wednesday that he wouldn’t reveal what steps Mexico would take if Texas attempts to return people over the border. But he suggested there could be retaliation: “We will not just sit around with our arms crossed.”