By Ted Hesson

WASHINGTON (Reuters) -U.S. immigrant rights groups sued the Biden administration on Wednesday over a new policy that bars most migrants caught crossing the U.S.-Mexico border illegally from claiming asylum, threatening to derail the far-reaching enforcement effort.

The groups, led by the American Civil Liberties Union, argued restrictions implemented last week violated U.S. asylum law and that Biden failed to follow proper regulatory procedure. The lawsuit was filed in federal court in Washington, D.C.

Biden, a Democrat seeking another term in Nov. 5 elections, has grappled with millions of migrants caught illegally crossing the southwest border during his presidency. Immigration is a top election issue and Biden’s Republican challenger, former President Donald Trump, has vowed to crack down aggressively if he wins another term in the White House.

Biden has toughened his stance in recent months, backing a bipartisan Senate bill that would surge resources to the border and expand enforcement capacity. The new ban mirrors Trump-era policies that sought to deny migrants access to asylum at the border and utilizes the same legal authority as Trump’s travel bans blocking people from majority-Muslim nations and elsewhere.

Lee Gelernt, deputy director of the ACLU’s Immigrants’ Rights Project, said the Biden administration “lacks unilateral authority to override Congress,” citing previous legal decisions related to Trump-era policies.

Migrants caught crossing illegally could be quickly deported or turned back to Mexico under the Biden measure. The ban included exceptions for unaccompanied children, people who face serious medical or safety threats and victims of trafficking.

Key operational questions remain unclear, including how the U.S. would quickly deport migrants from far-away and uncooperative countries and how many non-Mexican migrants Mexico would accept under the new enforcement regime.

(Reporting by Ted Hesson in Washington; Edited by Josie Kao and Daniel Wallis)