John McCain’s nomination as the Republican candidate may be an electoral near-certainty, but his campaign is investigating whether the senator’s birth in the Panama Canal Zone may disqualify him from the presidency.

Mr McCain was born in 1936 while his father was stationed at a US military base and the Canal Zone was under American control. Although the question was examined during his first presidential bid in 2000, it has been revived as the senator heads towards the nomination.

The issue has also revived a centuries-old debate about the exact meaning of a constitutional clause laid down by the founding fathers in 1787, which declares that only a “natural-born citizen” can occupy the Oval Office.

The restriction was most recently revisited over the possible candidacy of Arnold Schwarzenegger, the Governor of California who was born in Austria but has lived in the United States since 1968.

There is little guidance in the US Constitution as to how the provision should be interpreted and debate has frequently centred on whether only those born on US soil can be considered “natural-born”.

Many experts argue that the nation’s founders could never have intended to exclude the children of those serving in the military, but as all presidents to date have been born within the 50 states there is no legal precedent.

The lawyer Theodore Olson, a former Solicitor-General and adviser to the McCain campaign, said he was fairly certain of Mr McCain’s eligibility but had not yet completed his analysis.

However Mr Olson said the plain meaning of “natural-born citizen” included those born to parents who are citizens, particularly when they are born on a US military base. “I am confident that the United States Supreme Court, should it ever address the issue, would agree,” he added.

Mr McCain yesterday dismissed questions over his eligibility, and noted the case of the Republican Barry Goldwater, who was born in Arizona in 1909 – before it became a state – but ran for the presidency in 1964 and eventually lost to Lyndon Johnson.

“Barry Goldwater was born in Arizona when it was a territory, Arizona was a territory, and it went all the way to the Supreme Court,” he said. “And there’s no doubt about that. And it was researched again in 2000.”

“It’s very clear that (the idea that) an American born in a territory of the United States whose father is serving in the military would not be eligible for the presidency of the United States is certainly not something our founding fathers envisioned,” he insisted.

Jill Hazelbaker, a spokesman for the McCain campaign, said the request for legal help was purely routine, while The New York Times, the first mainstream news outlet to run the story, drew fire from conservative commentators for raising the issue at all.

At present, it appears Democrats have little interest in pursuing the matter.

Yesterday, Claire McCaskill, a senator from Missouri and a prominent backer of Democratic candidate Sen. Barack Obama, introduced legislation that would define a “natural-born citizen; as anyone born to any U.S. citizen while serving in the active or reserve components of the U.S. armed forces.

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