President Trump on Wednesday renewed his threat to cut off automatic birthright citizenship for babies born in the U.S. to illegal immigrant mothers.

“We are looking at birthright citizenship very seriously,” he told reporters at the White House.

He didn’t say what steps he is eyeing, and legal scholars say there are major hurdles — including the Constitution.

The 14th Amendment says “all persons born or naturalized in the United States and subject to the jurisdiction thereof are citizens of the United States.”

Though the Supreme Court has never ruled specifically on a modern case involving an illegal immigrant, and there is some gray area in the “subject to the jurisdiction” language, most legal scholars who have looked at the issue say they expect the courts would rule the Constitution does grant automatic citizenship, though a minority of scholars say there’s room to maneuver.

Mr. Trump last year said he was pondering an executive order to try to end the practice.

Nothing developed from that promise.

Those who want to see a crackdown on illegal immigration have long speculated that automatic citizenship is a magnet for some migrants to enter the U.S.

Officials in some border towns say they do see evidence of mothers entering the U.S. to give birth, and the federal Justice Department in January announced it had busted several massive “birth tourism” operations in California that catered to wealthy Chinese women.

The women were charged up to $100,000 and were coached on how to obtain a U.S. visitor’s visa, told how to fool government screeners, and were given space in apartments to await delivery of their children.