Joe Biden will introduce an immigration executive order that will severely restrict the flow of asylum seekers

President Joe Biden is expected to announce as early as Tuesday an executive order that would effectively close the U.S.-Mexico border to asylum seekers illegally crossing the border once they exceed the daily crossing threshold – a wide-ranging and controversial proposal that is likely to face backlash. Opposition from progressives and immigration advocates was strongly welcomed.

The executive action, described to CNN by multiple people briefed on the order, would mark a significant attempt by Biden to address one of his biggest policy weaknesses just weeks before his first presidential debate with former President Donald Trump. Trump has made tough immigration policy a cornerstone of his campaign.

The executive order uses a provision known as “212(f)” – a regulation used during the Trump administration and widely condemned by Democrats at the time.

The executive action, which has been in the works for months, would severely limit migrants’ ability to seek asylum at the U.S. southern border if they cross the border illegally once the daily number of encounters between ports of entry reaches 2,500, meaning the measure will likely take effect almost take effect immediately. Migrants will still be able to request an appointment to present their asylum claim at a port of entry.

Unaccompanied children would be exempt – a key element of the executive order that would alarm immigration advocates, who have argued that such an exemption could encourage some families to send their children to the border on their own.

The executive action would allow authorities to turn back migrants while in Mexico or their country of origin. The United States has previously worked with Mexico when a Covid-19-era border restriction, known as Title 42, also resulted in some migrants returning to Mexico.

Sources who described the order to CNN cautioned that changes could still be made to the text before it is publicly disclosed this week.

The action would mark a significant shift in policy from the Biden administration, which has taken a more aggressive approach to border security after years of grappling with border crises sparked by record migration in an attempt to blunt Republican attacks in a hot election year.

Trump campaign spokeswoman Karoline Leavitt sharply criticized Biden’s expected executive order on Tuesday, saying the president is “controlled by radical left-wing Democrats.” Trump tried to use similar powers to restrict border crossings while in office, but was ultimately blocked by a federal appeals court.

According to a homeland security official, border authorities recently apprehended nearly 4,000 migrants crossing the border illegally, and about 1,500 encounters a day are taking place through the U.S. Customs and Border Protection mobile app.

According to one of the sources, according to the regulation, the border will be reopened after daily illegal border crossings cease.

Senior White House officials, including Chief of Staff Jeff Zients, called lawmakers and other stakeholders over the weekend to inform them of the plans, according to two sources familiar with the discussions.

CNN was first to report that the White House had begun contacting border mayors asking them to participate in the announcement.

Edinburgh Mayor Ramiro Garza confirmed to CNN that the administration had contacted him on Saturday to attend Tuesday’s event at the White House. Brownsville Mayor John Cowen said he was also invited and will attend. Harlingen Mayor Norma Sepulveda and Laredo Mayor Dr. Victor D. Trevino also confirmed to attend the event. McAllen Mayor Javier Villalobos also said he was invited but would not be able to attend.

CNN’s Camila DeChalus contributed to this report.