Republican presidential candidate Donald Trump continued his push Monday to derail GOP rival Ted Cruz's rise in recent polls by raising questions about his citizenship, saying it could become a problem for the party if he wins the nomination.

"Whether you like it or not, Ted has to figure it out because ... you can't have the person that gets the nomination be sued like Hillary might be sued," Trump said during a campaign rally in Windham, N.H.

Trump has repeatedly gone after Cruz's eligibility to run for president in recent days, saying that his Canadian birth could become an issue Democrats will exploit in the general election. The Texas senator is leading the field in Iowa with just three weeks to go before the Feb. 1 Iowa caucuses.

The Cruz campaign has dismissed the comments repeatedly, saying that Cruz's mother was an American and that makes the Texas senator a natural citizen.

In Windham on Monday, Trump repeated his "advice" to Cruz that he seek a judicial declaration verifying his eligibility to run. He also indicated that the longer Cruz waits, the more serious the problem could become.

"Look, it doesn't matter what he does. You can't have a nominee who is going to be subject to be thrown out as nominee," Trump said. "You just can't do it. So you're going to have to make that decision, folks. It's one of those little decisions."

"I'm sure Ted is thrilled that I'm helping him out," he quipped. "But I am! I mean, I am!"

Trump also accused Cruz of following his lead on immigration policy by proposing to build a wall along the southern border. Trump has repeated that accusation several times in recent days during campaign stops across the country.

During the speech, the billionaire showman also use the unusual tactic of attacking former New Hampshire governor John H. Sununu, a GOP institution unto himself in the Granite State. Trump called Sununu "such a dumb guy" and saying "he was fired like a dog" by president George. H.W. Bush. Sununu has been highly critical of Trump's rise, accusing the candidate of lacking substance.

Perhaps most surprisingly, Trump himself came under fire from a heckler, who began shouting, "This is boring!" as Trump spoke.

That hit a soft spot.

"Get out of here, go ahead. Nothing is funny about this. He wants jokes!" Trump said, ordering security to throw out the heckler. "There's nothing funny about this, this is so serious."