WASHINGTON – President Donald Trump may have met his match Tuesday: the 14th Amendment to the U.S. Constitution.

"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 1868 amendment begins.

"It's ridiculous," Trump said 150 years later, on the eve of midterm elections that could erode his power, perhaps for the rest of his presidency. "And it has to end."

So began the 45th president's latest legal, policy and political battle, neatly contained in a history lesson dating to Reconstruction.

As the administration vowed to block a migrant caravan heading toward the southern border and delay a trial next week on its plan to ask about citizenship on the 2020 Census, Trump turned his focus on generations of American-born citizens.

In doing so, he took on both the Constitution and federal law. Title 8, Section 1401 of the U.S. Code lists those people deemed to be citizens of the United States, starting with "a person born in the United States and subject to the jurisdiction thereof."

The Supreme Court ruled in United States v. Wong Kim Ark, an 1898 case, that a man born on U.S. soil to parents who were Chinese nationals was a citizen. As part of a 1982 decision, Plyler v. Doe, the high court said that even if someone enters the country illegally, they are within U.S. jurisdiction, and their children born in the U.S. are entitled to the protections of the 14th Amendment.

Trump said he has been advised he can reverse both the Constitution and the law and end birthright citizenship by executive order. He has some support.

Michael Anton, former spokesman for the National Security Council, wrote in The Washington Post recently that the 14th Amendment was written so as not to apply to undocumented immigrants.

Because the amendment addresses people subject to U.S. jurisdiction, Anton wrote, it distinguishes between "people to whom the United States owes citizenship and those to whom it does not.

"Freed slaves definitely qualified," Anton said. "The children of immigrants who came here illegally clearly don't."

Other conservatives contend that at least Congress, if not the president, can end birthright citizenship by legislation. John Eastman, director of Claremont Institute's Center for Constitutional Jurisprudence, has written that the 1898 ruling applied only to the children of legal immigrants.

"It is long past time to clarify that the 14th Amendment does not grant U.S. citizenship to the children of anyone just because they can manage to give birth on U.S. soil," Eastman wrote in The New York Times.

Most legal scholars disagree. In testimony before Congress in 1995, assistant attorney general Walter Dellinger said even congressional efforts to deny citizenship to some children born in the U.S. were non-starters.

"No discretion should be exercised by public officials on this question – there should be no inquiry into whether or not one came from the right caste, or race, or lineage or bloodline in establishing American citizenship," Dellinger said. "In America, a country that rejected monarchy, each person is born equal, with no curse of infirmity, and with no exalted status, arising from the circumstance of his or her parentage."

On Tuesday, Dellinger noted that Trump’s proposal would question even the citizenship of people whose parents or grandparents had birthright citizenship.

Dellinger is a liberal, but many conservatives agree with his assessment. James Ho, named by Trump to the U.S. Court of Appeals for the 5th Circuit last year, wrote in 2011 that states' efforts to rewrite U.S. citizenship law was unconstitutional.

"Opponents of illegal immigration cannot claim to champion the rule of law and then, in the same breath, propose policies that violate our Constitution," Ho wrote. "Birthright citizenship is a constitutional right, no less for the children of undocumented persons than for descendants of passengers of the Mayflower."

Before the Civil War, the Constitution did not define explicitly what made someone a U.S. citizen, leading to decades of disputes, according to legal scholars Akhil Reed Amar and John C. Harrison. The Supreme Court had ruled in 1857's Dred Scott v. Sandford decision that people of African descent, whether slave or free, were not entitled to full citizenship.

"At the simplest level, the 14th Amendment's Citizenship Clause was meant to repudiate Dred Scott," Amar wrote. "However, it was also meant to root post-Civil War America – America’s Second Founding – in an inspiring Lincolnian reinterpretation of one of our nation’s Founding truths, that we’re all created/born free and equal."

Amar, a Yale University law professor, said the amendment's citizenship clause "marked an important shift in American identity" and "established a simple national rule for citizenship: If you’re born in America under our flag, you’re a U.S. citizen."

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