Business magnate Donald Trump has attacked his Republican opponent Ted Cruz by questioning the latter's eligibility to be a presidential candidate. The controversial real estate mogul, who trails Cruz in the state of Iowa, claimed that the Texas senator was born in Canada.

In a campaign stop at Clear Lake, Trump said that there was ambiguity on whether he meets the constitutional rights to be a "natural born citizen". "You can't have a person who's running for office, even though Ted is very glib and he goes out and says 'Well, I'm a natural-born citizen', but the point is you're not," Trump pointed out.

Cruz was born in Calgary, Canada, to a Cuban father but an American mother, which automatically conferred upon him the American citizenship. Most legal experts agree that satisfies the requirement to be a "natural-born citizen", a term which has not been "properly" defined to be eligible to contest US presidential polls.

Quoting constitutional expert Laurence Tribe, of Harvard University, Trump tweeted that the question "who is a natural-born citizen is not a settled matter". Incidentally, Tribe had taught Cruz Constitutional Law in Harward.

Moreover, Trump said that Cruz would have to go to court to get a "declaratory judgment" about his eligibility or "you have a candidate who just cannot run". The 45-year-old will have to obtain a judgment if someone filed a lawsuit to challenge his candidacy, according to the New York Times.

The billionaire raked up the "eligibility issue" as polls in Iowa showed neck-and-neck competition ​between the two contenders.​ Commenting on the issue, Cruz said: "Under longstanding federal law, the child of a U.S. citizen born abroad is a natural-born citizen." He was speaking on Saturday ahead of his final stop during a six-day bus tour of Iowa.

"You're seeing candidates trying to throw whatever rocks they can. That's fine, that's their prerogative. I like Donald Trump, I respect Donald Trump. He's welcome to toss whatever attacks he wants," he added.

"The only way to win Iowa is one voter at a time, showing the voters, the men and women of Iowa, the respect to look them in the eyes, to answer their questions. I don't believe any candidate will win the state of Iowa, and I don't believe any candidate will win the state of New Hampshire from a TV studio in Washington, D.C., or in Manhattan."