Trump is right about Cruz's presidential eligibility: the meaning of "natural born citizen" remains unsettled.

• Author: Sullum, Jacob
• Position: Columns - Donald Trump and Ted Cruz - Column

DURING THE REPUBLICAN presidential debate on January 14, Ted Cruz said it's "really quite clear" he is eligible to run for president even though he was born in Canada, because his mother was a U.S. citizen. His rival Donald Trump insisted "there is a serious question" as to whether Cruz qualifies as "a natural born citizen," one of the constitutional requirements for the presidency.

Here is a sentence I never thought I'd type: Donald Trump is right. Cruz sees a consensus that does not exist.

The Texas senator is not alone. In a 2015 Harvard Law Review essay, Neal Katyal and Paul Clement-solicitors general under Barack Obama and George W. Bush, respectively--say "there is no question that Senator Cruz has been a citizen from birth and is thus a 'natural born Citizen' within the meaning of the Constitution." They call claims to the contrary "specious" and "spurious."

No doubt Mary Brigid McManamon, a legal historian at Delaware Law School, would object to those adjectives. In a Washington Post op-ed piece published in January, she says it's "clear and unambiguous," based on British common law during the Founding era, that Cruz is not a "natural born citizen."

As Catholic University law professor Sarah Helene Duggin and Maryland lawyer Mary Beth Collins show in a 2005 Boston University Law Review article, these dueling perspectives are the latest installment of a long-running scholarly debate about the meaning of "natural born citizen." Contrary to Cruz, Katyal, Clement, and McManamon, Duggin and Collins view the phrase as "opaque" and dangerously "ambiguous" (as well as outdated, unfair, and antidemocratic), arguing that it should be excised by amendment.

Harvard law professor Laurence Tribe, whom Trump likes to cite, has taken both sides in this debate. In 2008 Tribe and former Solicitor General Ted Olson coauthored a memo that said John McCain, the GOP nominee that year, was eligible for the presidency even though he was born in the Panama Canal Zone.

Since the Constitution does not define "natural born citizen," Tribe and Olson wrote, to illuminate the term's meaning we must look to the context in which it is used, legislation enacted by the First Congress, and "the common law at the time of the Founding." They said "these...