The United States could bar tens of thousands of migrants arriving at the U.S.-Mexico border from applying for asylum under a proposal unveiled Tuesday that would be the most sweeping effort yet by Joe Biden’s administration to deter unauthorized crossings.

Under the new rules, the U.S. would generally deny asylum to migrants who show up at the U.S. southern border without first seeking protection in a country they passed through, reflecting an effort by Trump’s administration which never took effect because it was blocked in court.

The measure, while not a total ban, imposes severe restrictions on asylum for people of all nationalities except Mexicans, who do not need to travel through a third country to reach the United States.

The proposed rule establishes “a rebuttable presumption of asylum eligibility” for anyone who passes through another country to reach the U.S. border with Mexico without first seeking protection there, according to a notice in the Federal Register. Exceptions will be made for people with “urgent medical emergencies”, “imminent and extreme threats” of violent crimes such as murder, rape or kidnapping, being victims of human trafficking or “other extremely compelling circumstances”. Children traveling alone will also be exempt, according to the rule.

The measure, which was published online Tuesday, will be subject to a 30-day public comment period before it can be formally adopted. It would also be temporary and limited to a period of two years, with the option of extending it.

Biden, a Democrat who took office in 2021 and is expected to seek re-election in 2024, initially promised to restore access to asylum that was restricted under his Republican predecessor. But advocates and some other Democrats have criticized him for increasingly embracing Trump-style restrictions as he has struggled to cope with record migrant arrivals.

The measure is almost certain to face legal challenges; Donald Trump sought a similar ban in 2019 but a federal appeals court blocked it from taking effect.

The American Civil Liberties Union (ACLU) vowed to fight the Biden rule in court, comparing it to the Trump restriction, which was called a “transit ban” by activists.

“We successfully sued to block Trump’s transit ban and will sue again Biden administration is going through with its plan,” said Lee Gelernt, the ACLU attorney who argued the Trump-era lawsuit.

Karen Musalo, director of the Center for Gender and Refugee Studies at the University of California College of the Law, San Francisco, said the Biden proposal ignored dangerous conditions and limited asylum capacity in transit countries where migrants are expected to seek refuge.

“It’s a terrible example of trying to ignore your domestic and international legal obligations,” she said.

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The American Civil Liberties Union vowed to fight the Biden rule in court. Photo: Gerardo Vieyra/NurPhoto/REX/Shutterstock

The Biden administration began discussing the ban and other Trump-style measures last year as a way to reduce illegal crossings if pandemic-era restrictions allowing many migrants to be deported back to Mexico ended. The administration is moving forward with tougher asylum rules as the restrictions, known as Section 42, are likely to drop on May 11, when the Covid-19 public health emergency ends.

The Departments of Homeland Security and Justice argued that the rising number of migrants left them with no choice. They expect illegal crossings to climb to between 11,000 and 13,000 a day if no action is taken after Title 42 ends, an even higher figure than the 8,600 daily crossings in mid-December, as anticipation spread among migrants and smugglers that Section 42 was about to expire.

“Without a meaningful policy change, border encounters could increase and potentially increase dramatically” after the repeal of Title 42, the text of the proposed rule said.

Biden expanded Title 42 in January to deport additional nationalities while allowing some people from those countries to apply for legal entry by air via humanitarian parole if they have U.S. sponsors. The release program, for up to 30,000 Cuban, Haitian, Nicaraguan and Venezuelan migrants per month, would be one of the legal pathways the administration says would allow asylum seekers to circumvent the proposed restrictions.

Separately, migrants seeking asylum at the U.S.-Mexico border can schedule an appointment at a U.S. port of entry using an app called CBP One. But since the CBP One initiative launched in January, migrants say slots have filled up quickly.

Reuters and the Associated Press contributed reporting