The question has nagged at the parents of Americans born outside the continental United States for generations: Dare their children aspire to grow up and become president? In the case of Senator John McCain of Arizona, the issue is becoming more than a matter of parental daydreaming.

Mr. McCain’s likely nomination as the Republican candidate for president and the happenstance of his birth in the Panama Canal Zone in 1936 are reviving a musty debate that has surfaced periodically since the founders first set quill to parchment and declared that only a “natural-born citizen” can hold the nation’s highest office.

Almost since those words were written in 1787 with scant explanation, their precise meaning has been the stuff of confusion, law school review articles, whisper campaigns and civics class debates over whether only those delivered on American soil can be truly natural born. To date, no American to take the presidential oath has had an official birthplace outside the 50 states.

“There are powerful arguments that Senator McCain or anyone else in this position is constitutionally qualified, but there is certainly no precedent,” said Sarah H. Duggin, an associate professor of law at Catholic University who has studied the issue extensively. “It is not a slam-dunk situation.”

Mr. McCain was born on a military installation in the Canal Zone, where his mother and father, a Navy officer, were stationed. His campaign advisers say they are comfortable that Mr. McCain meets the requirement and note that the question was researched for his first presidential bid in 1999 and reviewed again this time around.

State Republicans say Baucus has become a full-time Washington, D.C., politician who no longer really lives in Montana. They say the issue will come up in the upcoming campaign.

Baucus now owns one-half of his mother’s Helena home, and Baucus and his wife, Wanda, are listed on the home’s title as owners, records show.

Baucus spokesman Barrett Kaiser said the senator has always considered the house where he grew up “home” and returns at least twice a month.

“He’s called it home for as long as he can remember,” Kaiser said. “It’s the house where his son comes to visit his grandmother and it’s likely where Max’s grandchildren will come, too.”

Chris Wilcox, executive director of the Montana Republican Party, said it’s wonderful that Baucus comes back to Montana to visit his mother, but said it’s not the same thing as actually living here.

“The rest of our congressional delegation still keep their lives here, their families, their business operations” he said. “I think that’s an important difference.”