Donald Trump. REUTERS/Brian Snyder Real-estate mogul Donald Trump clearly thinks he has a winning issue against Sen. Ted Cruz (R-Texas) in the Republican presidential primary.

Indeed, Trump has repeatedly and increasingly brought up Cruz's supposed citizenship "problem" during his daily rallies.

Trump says Cruz could have difficulty meeting the constitutional requirement that presidents are "natural-born" citizens. Most legal experts believe Cruz's American mother is enough to qualify him for the presidency, as Cruz was a US citizen at birth.

However, the courts have never ruled on the issue, and Trump argues that this ambiguity should be deeply concerning to voters.

"Ted Cruz has a problem. I mean, he's got a problem," Trump said Monday at a rally in Windham, New Hampshire.

Trump warned that Democrats would launch a lawsuit questioning Cruz's eligibility if the senator were to secure the Republican presidential nomination. Trump compared that potential lawsuit to email controversy dogging former Secretary of State Hillary Clinton, the Democratic front-runner.

"Whether you like it or not, Ted has to figure it out. Because we can't be having a nominee — if he got the nod, I think I'm going to win very solidly, if you want to know the truth — but, if you get the nomination, you can't have the person who gets the nomination be sued," Trump said. "Like Hillary might be sued ... she'll be sued over the emails."

Trump went on to say "nobody knows" if Cruz would be allowed to become president. The Republican front-runner cited left-leaning legal scholar Laurence Tribe to make his point.

"Ted Cruz has a problem. Because the question is: Is he an actual-born citizen? ... I said, 'I don't know.' I mean, nobody knows," he continued. "And Laurence Tribe of Harvard — who is a constitutional expert, one of the best in the country — said, and I wrote it down: 'This is not a settled matter. It's wrong to say it is a settled matter.' Because it's absolutely not, it's not a settled matter. That means a lot of people think you have to be born here."

Trump added that the Republican Party "just can't" nominate someone in Cruz's position.

"In the case of Ted, he has to figure it out," he said. "Now, I think he can go in for some kind of a judiciary proceeding. Look, it doesn't matter what he does. You can't have a nominee who's going to be subject to be thrown out as the nominee. You just can't do it."

Cruz, who has long refused to directly criticize Trump, dismissed the mogul's attacks last week by arguing that he is clearly a natural-born citizen.