In congressional debates about the 14th Amendment in 1866, lawmakers said its sweep should be wide.

“Is the child of the Chinese immigrant in California a citizen?” Senator Edgar Cowan of Pennsylvania asked on the Senate floor.

Senator John Conness of California said the answer was yes.

“The children of all parentage whatever, born in California, should be regarded and treated as citizens of the United States, entitled to equal civil rights with other citizens,” Mr. Conness said.

The Supreme Court confirmed that understanding in 1898 in United States v. Wong Kim Ark, ruling that a child born in San Francisco to Chinese parents was a United States citizen, even though the parents were prohibited by the Chinese Exclusion Act from ever becoming citizens.

“To hold that the 14th Amendment of the Constitution excludes from citizenship the children, born in the United States, of citizens or subjects of other countries,” the court said, “would be to deny citizenship to thousands of persons of English, Scotch, Irish, German, or other European parentage who have always been considered and treated as citizens of the United States.”

James C. Ho, a conservative legal scholar recently appointed by Mr. Trump to the federal appeals court in New Orleans, has written that the message of the decision is unmistakable. “This sweeping language reaches all aliens regardless of immigration status,” he wrote in 2006.

The 1898 decision did not specifically discuss unauthorized immigrants. But in 1982, in Plyler v. Doe, the Supreme Court ruled that undocumented children were entitled to free public education. The court relied on another part of the 14th amendment, its equal protection clause, and it interpreted language similar to that in the citizenship clause.

“Although the court splintered over the specific question of public education,” Mr. Ho wrote, “all nine justices agreed that the Equal Protection Clause protects legal and illegal aliens alike. And all nine reached that conclusion precisely because illegal aliens are ‘subject to the jurisdiction’ of the U.S., no less than legal aliens and U.S. citizens.”