A growing number of constitutional law scholars are arguing that Ted Cruz’s birth in Canada makes him ineligible to become U.S. president. Their argument could prove a thorn in the side of the senator, who is a zealous originalist on most constitutional questions—with what seems like a notable exception.

The issue has moved to the center of the presidential campaign, with Cruz’s rise in the polls and Donald Trump claiming that Cruz needs to prove he’s eligible to run by getting a declaratory judgment in federal court.

There is some ambiguity in the question of eligibility. The Constitution sets down three requirements to assume the nation's highest office: one must be at least 35 years old, have been a resident of the U.S. for at least 14 years (though whether those years must be consecutive or can be cumulative is a question up for debate) and must be a "natural-born citizen" of the United States. But the founders did not explicitly define "natural-born citizen," leaving room for doubt and debate.