When Arizona Sen. John McCain, R, was running for president in 2008, his campaign experienced turbulence over an issue currently vexing the campaign of Texas Sen. Ted Cruz: the phrase in the Constitution stipulating that the president must be a “natural born citizen.”

There are conflicting legal views (naturally) about what that means. Law professor Gabriel Chin of the University of California at Davis wrote a paper arguing that McCain’s birth outside U.S. borders, in the Canal Zone, made him ineligible for the presidency. Billionaire Donald Trump, a birther’s birther, is now making a similar claim about Cruz, who was born in Canada and held Canadian citizenship for most of his life.

I asked Trevor Potter, a former Federal Elections Commission chairman who was the general counsel of McCain’s presidential campaign, about that campaign’s research into McCain’s eligibility, and what it might tell us about Cruz’s situation. McCain is no champion of Cruz. But Potter studied the issue carefully. Bottom line: It’s dicier than you might think.

Q: How seriously did you take claims that John McCain might not qualify as a “natural born” citizen?

A: We looked carefully into the issue early on. We were aware of the question being raised in earlier presidential campaigns. Barry Goldwater was born in a “territory” of the U.S., and George Romney was born in Mexico to two American-born parents.

We consulted constitutional law professors. As you know, we had an opinion from [Gibson Dunn lawyer Theodore] ‘Ted’ Olson and [Harvard professor Laurence] ‘Larry’ Tribe that confirmed our understanding that John McCain’s birth — to two American parents on a U.S. military base in a U.S.-controlled territory – – was as close to the historic English understanding of the term “natural born citizen” as possible.

Q: Those are a lot of supporting factors: U.S. parents on a U.S. military base in a U.S.-controlled territory. Despite all that, the McCain legal team was still concerned about the definition of “natural born citizen” and whether a legal challenge to a McCain presidency might be upheld?

A: Given the results of the research, we were very comfortable that we would win any legal challenge. What was less clear is who would have standing to bring a legal challenge, and at what stage in the process. The campaign actually responded to several suits, and all were dismissed by the courts.

Q: Would you have the same degree of confidence if your candidate — just thinking out of the blue here — had been born in Canada of one American parent?

A: We believed that Senator McCain’s birth made his situation absolutely congruent with English precedents at the time the Constitution was adopted. The founders used the English phrase “natural born citizen,” which had that same common meaning. Any divergence from Senator McCain’s particular circumstances by definition changes the facts, and takes us further away from the common law meaning circa 1789.

One American parent and one foreign parent; a birth in a foreign country and not on a U.S. base; and not while the parents were in the service of the nation; dual citizenship for an entire adult life — all of those facts are certainly different from Senator McCain’s case.

Q: So is it fair to say that you were comfortable with McCain’s circumstances in 2008 but you would not be able to apply your conclusions from that experience to the very different circumstances of Senator Cruz?

A: After conducting our legal analysis of the term “natural born citizen” we were very comfortable with Senator McCain’s eligibility based on multiple factors. Without those specific factors — two U.S. citizen parents, birth on a U.S. base on U.S.-controlled territory — our comfort level that the candidate met the constitutional requirement would have declined.

 Francis Wilkinson writes on politics and domestic policy for Bloomberg View.

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