The United States could bar tens of thousands of migrants arriving at the U.S.-Mexico border from claiming asylum under a proposal unveiled on Tuesday that would be the most wide-ranging attempt yet by U.S. President Joe Biden's administration to deter people from crossing the border without authorization.

Under the new rules, migrants who fail to use existing legal pathways for U.S. entry or seek protection in countries they pass through en route to the United States would automatically be considered ineligible for asylum unless they qualify for certain exceptions.

Reuters first reported details of the measure, which was posted online on Tuesday and will be subject to a 30-day public comment period before being reviewed for final publication.

Biden, a Democrat who took office in 2021 -- and is expected to seek re-election in 2024 -- initially pledged to restore asylum access that was curtailed under his Republican predecessor, Donald Trump. But advocates and some fellow Democrats have criticized him for increasingly embracing Trump-style restrictions as he has struggled to cope with record numbers of arriving migrants.

Biden's plan to ban certain asylum seekers mirrors similar efforts under Trump that were blocked by federal courts.

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

"We successfully sued to block the Trump transit ban and will sue again if the Biden administration goes through with its plan," said Lee Gelernt, the ACLU attorney who argued the Trump-era lawsuit.

Families and single adults will be subject to the restrictions while unaccompanied minors will be exempt, according to the text of the rule, issued jointly by the U.S. Department of Homeland Security (DHS) and U.S. Department of Justice (DOJ). The measure would be temporary and time-limited to a period of two years.

The Biden administration began discussing the ban and other Trump-style measures last year as a way to reduce illegal crossings if COVID-era restrictions allowing many migrants to be expelled back to Mexico were ended. The administration is moving ahead with tougher asylum rules as the COVID restrictions, known as Title 42, appear likely to sunset on May 11 when the COVID-19 public health emergency terminates.

"Without a meaningful policy change, border encounters could rise, and potentially rise dramatically" after the lifting of Title 42, the text of the rule said, estimating crossings could reach up to 13,000 per day without the COVID restrictions, up from an average of about 5,000 a day in January.

A Biden administration official, who declined to be named, told reporters on a call on Tuesday that the rule "is intended to fill the void that Congress has left by taking no action," to pass meaningful immigration reform.

Mexican authorities did not immediately respond to a request for comment.