President Trump plans to sign an executive order that would remove the right to citizenship for babies of non-citizens and unauthorized immigrants born on U.S. soil, he said yesterday in an exclusive interview for "Axios on HBO," a new four-part documentary news series debuting on HBO this Sunday at 6:30 p.m. ET/PT.

Why it matters: This would be the most dramatic move yet in Trump's hard-line immigration campaign, this time targeting "anchor babies" and "chain migration." And it will set off another standoff with the courts, as Trump’s power to do this through executive action is debatable, to say the least.

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Trump told "Axios on HBO" that he has run the idea of ending birthright citizenship by his counsel and plans to proceed with the highly controversial move, which certainly will face legal challenges.

"It was always told to me that you needed a constitutional amendment. Guess what? You don't," Trump said, declaring he can do it by executive order .

that you needed a constitutional amendment. Guess what? You don't," Trump said, declaring he can do it by executive order When told that's very much in dispute, Trump replied: "You can definitely do it with an Act of Congress. But now they're saying I can do it just with an executive order."

very much in dispute, Trump replied: "You can definitely do it with an Act of Congress. But now they're saying I can do it just with an executive order." "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," Trump continued. "It's ridiculous. It's ridiculous. And it has to end." (More than 30 countries, most in the Western Hemisphere, provide birthright citizenship.)

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," Trump continued. "It's ridiculous. It's ridiculous. And it has to end." (More than 30 countries, most in the Western Hemisphere, provide birthright citizenship.) "It's in the process. It'll happen ... with an executive order."

The president expressed surprise that "Axios on HBO" knew about his secret plan: "I didn't think anybody knew that but me. I thought I was the only one. "

Behind the scenes: "Axios on HBO" had been working for weeks on a story on Trump’s plans for birthright citizenship, based on conversations with several sources, including one close to the White House Counsel’s office.

The legal challenges would force the courts to decide on a constitutional debate over the 14th Amendment, which says:

"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."

Be smart: Few immigration and constitutional scholars believe it is within the president's power to change birthright citizenship, former U.S. Citizenship and Immigration Services chief counsel Lynden Melmed tells Axios.

But some conservatives have argued that the 14th Amendment was only intended to provide citizenship to children born in the U.S. to lawful permanent residents — not to unauthorized immigrants or those on temporary visas.

that the 14th Amendment was only intended to provide citizenship to children born in the U.S. to lawful permanent residents — not to unauthorized immigrants or those on temporary visas. John Eastman, a constitutional scholar and director of Chapman University's Center for Constitutional Jurisprudence, told "Axios on HBO" that the Constitution has been misapplied over the past 40 or so years. He says the line "subject to the jurisdiction thereof" refers to people with full, political allegiance to the U.S. — such as green card holders and citizens.

Michael Anton, a former national security official in the Trump administration, recently took up this argument in the Washington Post.

Anton said that Trump could, via executive order, "specify to federal agencies that the children of noncitizens are not citizens" simply because they were born on U.S. soil. (It’s not yet clear whether Trump will take this maximalist argument, though his previous rhetoric suggests there’s a good chance.)

that Trump could, via executive order, "specify to federal agencies that the children of noncitizens are not citizens" simply because they were born on U.S. soil. (It’s not yet clear whether Trump will take this maximalist argument, though his previous rhetoric suggests there’s a good chance.) But others — such as Judge James C. Ho, who was appointed by Trump to Fifth Circuit Court of Appeals, in New Orleans — say the line in the amendment refers to the legal obligation to follow U.S. laws, which applies to all foreign visitors (except diplomats) and immigrants. He has written that changing how the 14th Amendment is applied would be "unconstitutional."

Between the lines: Until the 1960s, the 14th Amendment was never applied to undocumented or temporary immigrants, Eastman said.

Between 1980 and 2006, the number of births to unauthorized immigrants — which opponents of birthright citizenship call "anchor babies" — skyrocketed to a peak of 370,000, according to a 2016 study by Pew Research. It then declined slightly during and following the Great Recession.

the number of births to unauthorized immigrants — which opponents of birthright citizenship call "anchor babies" — skyrocketed to a peak of 370,000, according to a 2016 study by Pew Research. It then declined slightly during and following the Great Recession. The Supreme Court has already ruled that children born to immigrants who are legal permanent residents have citizenship. But those who claim the 14th Amendment should not apply to everyone point to the fact that there has been no ruling on a case specifically involving immigrants in the country illegally or those with temporary legal status.

The bottom line: If Trump follows through on the executive order, "the courts would have to weigh in in a way they haven't," Eastman said.

The full interview will air on "Axios on HBO" this Sunday, Nov. 4, at 6:30 p.m. ET/PT.

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