Ms. Duggin and others who have explored the arcane subject in depth say legal argument and basic fairness may indeed be on the side of Mr. McCain, a longtime member of Congress from Arizona. But multiple experts and scholarly reviews say the issue has never been definitively resolved by either Congress or the Supreme Court.

Ms. Duggin favors a constitutional amendment to settle the matter. Others have called on Congress to guarantee that Americans born outside the national boundaries can legitimately see themselves as potential contenders for the Oval Office.

“They ought to have the same rights,” said Don Nickles, a former Republican senator from Oklahoma who in 2004 introduced legislation that would have established that children born abroad to American citizens could harbor presidential ambitions without a legal cloud over their hopes. “There is some ambiguity because there has never been a court case on what ‘natural-born citizen’ means.”

Mr. McCain’s situation is different from those of the current governors of California and Michigan, Arnold Schwarzenegger and Jennifer M. Granholm, who were born in other countries and were first citizens of those nations, rendering them naturalized Americans ineligible under current interpretations. The conflict that could conceivably ensnare Mr. McCain goes more to the interpretation of “natural born” when weighed against intent and decades of immigration law.

Mr. McCain is not the first person to find himself in these circumstances. The last Arizona Republican to be a presidential nominee, Barry Goldwater, faced the issue. He was born in the Arizona territory in 1909, three years before it became a state. But Goldwater did not win, and the view at the time was that since he was born in a continental territory that later became a state, he probably met the standard.

It also surfaced in the 1968 candidacy of George Romney, who was born in Mexico, but again was not tested. The former Connecticut politician Lowell P. Weicker Jr., born in Paris, sought a legal analysis when considering the presidency, an aide said, and was assured he was eligible. Franklin D. Roosevelt Jr. was once viewed as a potential successor to his father, but was seen by some as ineligible since he had been born on Campobello Island in Canada. The 21st president, Chester A. Arthur, whose birthplace is Vermont, was rumored to have actually been born in Canada, prompting some to question his eligibility.

Quickly recognizing confusion over the evolving nature of citizenship, the First Congress in 1790 passed a measure that did define children of citizens “born beyond the sea, or out of the limits of the United States to be natural born.” But that law is still seen as potentially unconstitutional and was overtaken by subsequent legislation that omitted the “natural-born” phrase.