WASHINGTON — President Donald Trump fulfilled his midterm promise to crack down on undocumented immigrants crossing the Southwest border on Friday by signing a proclamation that will make it harder for immigrants to claim asylum if they are caught crossing the border between designated ports of entry.

Starting just after midnight on Saturday morning, asylum-seekers who do not go through ports of entry will be apprehended, detained and deported unless they can meet a higher bar, such as proving they would be tortured if they were sent home.

Within hours, the American Civil Liberties Union sued to block the new restrictions, calling it "the asylum ban."

"The asylum ban is not just unlawful but will put families in real danger. We hope that the courts will step in to remedy this injustice," said Lee Gelernt, deputy director of the ACLU's Immigrants' Rights Project.

Senior administration officials told reporters on a conference call Thursday that the president has the legal authority to do so because of sections of immigration law that allow the president discretion over who is admitted into the United States — the same language the administration used to support its travel ban in court.

The officials said the goal is to force more immigrants who wish to claim asylum to do so at designated ports of entry. Recently, many asylum-seekers have chosen to cross illegally because they are kept waiting for days in Mexico due to backlogs at ports of entry.

The new restrictions will apply for 90 days, but could end earlier, officials said.

Under international law, however, asylum-seekers are permitted to make a claim regardless of where they enter.

The administration was expecting lawsuits to be filed, which could keep the new policy from going into effect, but officials told NBC News that they expect the Supreme Court will eventually uphold their right to block asylum seekers with Justice Brett Kavanaugh now on the bench.

Immigrants crossing between ports of entry will still be allowed to claim asylum, but will have to prove that they meet a higher bar than a "credible fear" of returning to their home country, the current preliminary test. Under the new rule, the officials said, asylum-seekers will only be permitted to remain at large in the U.S. as they await a court hearing if they can prove "reasonable fear" or that they are protected under the U.N.'s Convention against Torture.

The full text of the new regulation can be found here.