Donald Trump. Scott Olson/Getty Images Republican presidential front-runner Donald Trump polled his audience members at a Saturday event on which of his two favorite attack lines was more effective against Sen. Ted Cruz (R-Texas).

Trump particularly delights in questioning whether Cruz's birth in Canada could disqualify him from the presidency.

"He was a citizen of Canada. I called him an 'anchor baby' the other day, right?" he told his supporters in Clinton, Iowa.

"Anchor baby" is a derogatory term for birth tourism, or people entering the US for the purpose of having a child born on US soil, which grants automatic citizenship.

The real-estate mogul also pounced after The New York Times reported this month that Cruz didn't report two loans used to finance his 2012 Senate race in Texas. Cruz dismissed the issue as a paperwork error.

"Let me ask you something: Which is a bigger problem, the Wall Street or the anchor baby?" Trump said as his audience gave mixed signals.

"What do you think? You think Wall Street?" he continued. "I'd be curious because a poll came out and said Wall Street's a bigger problem than Canada, that he was born in Canada. I would have said Canada is a bigger problem, personally."

He then raised the Canada and bank-loan issues separately to gauge his audience's reaction.

"Just out of curiosity, ready: I'm going to go Wall Street first. I'm going to go Canada second, all right? Which is a bigger problem," Trump asked.

"Wall Street?" he offered, pausing for effect.

"Canada?" he inquired, eliciting more cheers.

"Yeah, I'm telling you. I'm telling you: The poll has it wrong. ... I just think the Canada thing is a very big problem," he reflected.

Cruz was born in Canada to an American mother, and most legal experts believe he meets the Constitution's requirement that presidents are "natural-born" citizens because the senator was a US citizen at the time of his birth. But as Cruz surged over the past month to challenge Trump's dominance in Monday's Iowa caucus, Trump has questioned Cruz's eligibility on a daily basis.

"Ted has a couple of things. No. 1: He was born in Canada, OK. I think that's it's a problem, all right? I think it's an a problem," Trump said Saturday. "You want to have a cloud over your head? ... You got a cloud. It's called a cloud. He's got a cloud. You don't know that he can even take office. You don't know that he can run," he added.

Trump further claimed that "many great lawyers have said, 'No, it is settled law. He cannot run.'"

That could be a reference to a January op-ed from a law professor at Widener University’s Delaware Law School. Trump previously hailed the professor as "highly respected."

The Cruz campaign responded to Trump's remarks by mocking him on Twitter: