WASHINGTON — President Joe Biden is expected to sign an executive action Tuesday that would temporarily shut down the southern border when daily migrant crossings between legal ports of entry exceed 2,500, with the border reopening when the number falls below 1,500, according to three people familiar with the discussions.

The White House has conveyed this threshold to lawmakers, two of the sources said, but stressed some details were still being finalized before Tuesday’s anticipated rollout.

Daily encounters at the U.S.-Mexico border are currently averaging more than 4,000, according to Department of Homeland Security officials, which suggests a shutdown could go into effect immediately, though administration officials are bracing for legal challenges.

Details of the executive order were reported exclusively by NBC News on May 23. A critical consideration was working with the administration of Mexican President Andrés Manuel López Obrador to get its cooperation on some key provisions, according to multiple officials familiar with the negotiations. On Sunday, Claudia Sheinbaum, whose candidacy was supported by López Obrador, won election to succeed him as president.

The executive action is expected to rely on a presidential authority in the U.S. Code known as Section 212 (f), which would let the president unilaterally “suspend the entry” of specific groups of migrants whenever the number of attempted border crossings grows too high.

A shutdown would not block trade, travel or entry by immigrants presenting themselves lawfully for asylum at ports of entry. But it would block migrants from applying for asylum if they crossed the border between ports of entry during the shutdown.

DHS is also expected to redesignate Temporary Protected Status (TPS) for Haiti, which would enable certain Haitian nationals who are in the U.S. to remain until conditions improve in Haiti, three of the sources said.

The country has been largely in control of armed gangs since March, and many lawmakers on Capitol Hill have urged the Biden administration to extend TPS to Haitians and halt deportations.

In recent days, the White House has reached out to mayors of border cities to attend the signing of the executive order, according to a source familiar with the matter.

Border politics have been a hot topic for months, with Senate Democrats’ recent May effort to advance a bipartisan border security bill hitting a wall.

In February, senators released details of a bipartisan agreement that would have imposed tougher immigration and asylum laws and allowed Biden to “shut down the border when it becomes overwhelmed,” the president explained in January remarks urging lawmakers to pass the bill.

But just days later, Republican senators killed the bill their own party had negotiated with Democrats.

Border security appears to be a weak spot for Biden, according to polling. Former President Donald Trump has a 30-point edge with registered voters on the question of which candidate would handle immigration and border security better, including a 23-point edge among Latino voters, according to a late-March CNBC national poll.

If elected, Trump has vowed to launch the biggest “domestic deportation operation” in his second term. During his administration, Trump made it harder for foreign-born workers to enter the U.S., with visa denials rising by over 20% in both fiscal year 2018 and 2019 and refugee admissions decreasing.

There were 179,725 encounters along the southern border in April, a slight decrease from recent months, according to U.S. Customs and Border Protection. Over 1.5 million encounters have been recorded this fiscal year to date, meaning fiscal year 2024 has so far outpaced fiscal years 2023, 2022 and 2021 in encounters, according to the data.

This article was originally published on NBCNews.com