By Ted Hesson and Steve Holland

WASHINGTON (Reuters) – U.S. President Joe Biden is expected to sign a sweeping new border measure on Tuesday that would allow authorities to quickly deport or send back to Mexico migrants caught crossing the southwest border if illegal entries surpass a certain level, according to two sources with knowledge of the move.

The measure, which would restrict access to asylum, would take effect when U.S. Border Patrol apprehensions surpass 2,500 per day, the two sources said. The illegal crossings would have to dip below 1,500 per day for the asylum restrictions to be lifted, one of the sources said.

The restrictions are not expected to apply to unaccompanied minors. The White House did not immediately respond to a request for comment.

The election-year move is expected to trigger legal challenges from immigrant and civil rights groups who have criticized Biden, a Democrat, for adopting hardline policies that mirror those of his Republican predecessor, former President Donald Trump.

Biden has toughened his approach to border security as immigration has emerged as a top issue for voting-age Americans in the run-up to Nov. 5 elections where he will face Trump in a rematch of the 2020 contest.

Biden took office in 2021 vowing to reverse some of Trump’s restrictive policies but grappled with record levels of migrants caught crossing illegally. Trump has criticized Biden for rolling back his policies and vowed a wide-ranging crackdown if reelected.

The new U.S. restrictions mirror a Biden-backed Senate bill that aimed to block migrants from claiming asylum if the number of migrants caught crossing illegally reached a certain level. The bill was crafted by a bipartisan group of senators but Republicans rejected it after Trump came out in opposition.

The number of migrants caught crossing the U.S.-Mexico border illegally dropped in recent months, a trend U.S. officials partly attribute to increased Mexican enforcement.

Claudia Sheinbaum was elected as Mexico’s first female president in a landslide victory on Sunday and will take office on Oct. 1. Biden’s border restrictions could put pressure on Sheinbaum, the successor to current President Andres Manuel Lopez Obrador, to keep illegal border crossings down.

(Reporting by Ted Hesson and Steve Holland in Washington; Editing by Mica Rosenberg and Alistair Bell)