Professor Chin wrote that simply being born in the Canal Zone did not satisfy the 14th Amendment, which says that “all persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States.”

Image A 1936 picture shows the future presidential candidate John McCain in the arms of his grandfather John Sidney McCain in the Canal Zone. His father, John Sidney McCain Jr., is at left. Credit... McCain Presidential Campaign, via Associated Press

A series of early-20th-century decisions known as the Insular Cases, he wrote, ruled that unincorporated territories acquired by the United States were not part of the nation for constitutional purposes. The Insular Cases did not directly address the Canal Zone. But the zone was generally considered an unincorporated territory before it was returned to Panama in 1999, and some people born in the Canal Zone when it was under American jurisdiction have been deported from the United States or convicted of being here illegally.

The second way Mr. McCain could have, and ultimately did, become a citizen was by statute, Professor Chin wrote. In Rogers v. Bellei in 1971, the Supreme Court said Congress had broad authority to decide whether and when children born to American citizens abroad are citizens.

At the time of Mr. McCain’s birth, the relevant law granted citizenship to any child born to an American parent “out of the limits and jurisdiction of the United States.” Professor Chin said the term “limits and jurisdiction” left a crucial gap. The Canal Zone was beyond the limits of the United States but not beyond its jurisdiction, and thus the law did not apply to Mr. McCain.

In 1937, Congress addressed the problem, enacting a law that granted citizenship to people born in the Canal Zone after 1904. That made Mr. McCain a citizen, but not one who was naturally born, Professor Chin said, because the citizenship was conferred after his birth.

In his paper and in an interview, Professor Chin, a registered Democrat, said he had no political motive in raising the question.

In March, Laurence H. Tribe, a law professor at Harvard and an adviser to Senator Barack Obama, prepared a memorandum on these questions with Theodore B. Olson, a former solicitor general in the Bush administration. The memorandum concluded that Mr. McCain is a natural-born citizen based on the place of his birth, the citizenship of his parents and their service to the country.