But according to the vast majority of leading legal scholars, it is Trump who should brush up on the subject. Whatever he’s been told by his advisors, the president doesn’t have the authority to repeal birthright citizenship by executive order, and there is essentially no controversy about that, experts say.

What is the ius soli?

The concept of ius soli (right of land), as opposed to ius sanguinis (right of blood), is the right to get citizenship based on the country where one is born, rather than through family relations. Trump lied when he said the US is the only country to grant birthright citizenship: There are 35 countries in the world that observe unrestricted ius soli. Many of them are in the Americas, where governments adopted the practice in order to attract residents from other places. Another 25 countries grant birthright citizenship under specific conditions, such as being born to permanent residents. However, some countries, including France, England, and India, have ratcheted back or abolished birthright citizenship in the past few decades.

In the US, birthright citizenship was introduced in the first section of the 14th amendment, ratified in 1868. It was created to address a specific group of people who were born on American soil: former slaves and their children after the Civil War. At the time, southern states opposed the amendment, but it still passed with a two-thirds majority.

The amendment states:

All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside.

The Trump argument

Trump hasn’t offered many details on his plans. He didn’t specify whether he means to stop birthright citizenship only for the children of undocumented immigrants or for all children of non-US citizens, regardless of whether they are in the country legally. “We’re the only country in the world where a person comes in and has a baby, and the baby is essentially a citizen of the United States… with all of those benefits. It’s ridiculous. It’s ridiculous. And it has to end,” he said in an interview with Axios released earlier this week.

He also hasn’t clarified how he got the impression that he can do away with ius soli via executive order, though it seems to be based on an unconventional interpretation of the 14th Amendment.

The legal scholars that he appears to refer to in his tweet are actually just a handful, says Garrett Epps, a professor of constitutional law at the University of Baltimore. Two of them—Peter Schuck of Yale University and Rogers Smith of the University of Pennsylvania—are well respected. The rest don’t enjoy much recognition within academia, adds Epps.

Under their interpretation, the clause “subject to the jurisdiction of thereof” does not apply to undocumented immigrants or their children, because they didn’t exist at the time the it was written, they argued earlier this year in an article in National Affairs magazine. But unlike Trump, who wants to address the question with a presidential order, they say the issue should be settled by Congress.

Under the best reading of the Citizenship Clause of the 14th Amendment, the citizenship status of the American-born children of illegal immigrants is not mandated by the Constitution. Rather, this clause empowers Congress to decide the matter in its policy discretion (so long as it does not violate other constitutional rights), thereby giving specific content to the principle of popular consent—perhaps the fundamental principle of American democracy—that the clause adopted.

The Ryan argument

Like Ryan, most legal scholars say there’s no room for that interpretation in the 14th Amendment. To them, put simply, it “means what it says,“ says Epps. Even James Ho, a federal judge appointed by Trump, stated that birthright citizenship is guaranteed to the children of immigrants under the amendment in a 2006 article (pdf) published in The Green Bag, a law journal. He went further, saying that its “sweeping language reaches all aliens regardless of immigration status.”

The issue of whether children of immigrants should be granted citizenship at birth has already been debated and settled in the Supreme Court. In 1989, it established that the US-born son of Chinese immigrants who were not citizens had the right to citizenship. The court didn’t discuss the issue of illegal immigration—the Constitution itself doesn’t address immigration status, Ho notes. But he points out there are other legal precedents that show US jurisdiction does apply to all people living on the US, regardless of their country of origin. This means that birthright citizenship “is protected no less for children of undocumented persons than for descendants of Mayflower passengers,” he wrote.

Can Trump still end of birthright citizenship?

He can try. He could write up an executive order, arguing that the 14th Amendment doesn’t apply to the children of foreign citizens. Like some of his other executive orders, it would likely be challenged in court.

The prospects of that argument prevailing should be slim, says Anil Kalhan, a professor of immigration law at Drexel University. Even in a Supreme Court with a conservative majority there should be little disagreement about the meaning of the amendment in question—if the justices are doing their job.

This doesn’t mean the court can’t ultimately decide to go with the fringe interpretation favored by Trump, Kalhan said, but it would be a political move, and an overstepping of the boundaries. “It would not be judicially sound,” he said.

But while Trump’s argument has no legal basis, it clearly has political capital, says Kalhan. The fact that the president’s claims managed to turn a settled legal issue in a controversy is evidence of that.

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