A 1937 law granting citizenship to children of U.S. parents in the Panama Canal Zone came too late to apply to McCain, the U.S. senator from Arizona and likely Republican presidential nominee, The New York Times reported. The Constitution requires the president be a "natural-born citizen," but the law made McCain a citizen just before his first birthday.

"It's preposterous that a technicality like this can make a difference in an advanced democracy," professor Gabriel Chin said. "But this is the constitutional text that we have."

The law at the time of McCain's birth conferred citizenship to children born to American parents "out of the limits and jurisdiction of the United States," Chin said. The term "limits and jurisdiction" created a problem because the Canal Zone was beyond the limits of the United States but not its jurisdiction, the Times said.

McCain has dismissed suggestions that he does not meet the citizenship test. In April, the Senate approved a non-binding resolution declaring McCain eligible to be president.

"No court will get close to it, and everyone else is on board, so there's a constitutional consensus, the merits of arguments such as this one aside," said Peter Spiro, a citizenship law authority at Temple University.

A lawsuit challenging McCain's qualifications is pending in a federal court in Concord, N.H.