Just days away from the nation's kickoff nominating contest in Iowa, Donald Trump isn't sweating his slight lead over rival Ted Cruz.

"You know, I've gone up very much in Iowa," Trump told CBS' "Face the Nation" in an interview to air Sunday. "He's gone down very much in Iowa. But I'll tell you what -- you can't nominate somebody that maybe is not going to be able to run."

Trump was referring to the citizenship question that has dogged Cruz -- who was born in Canada to an American mother and a Cuban father -- ever since Trump saw fit to hammer away at the issue earlier this year.

"There is much law right now, and a lot of constitutional lawyers are coming out," the billionaire said. "There are many lawyers coming out, top constitutional lawyers, that Ted Cruz cannot run for president. He can't be president, because of the fact he was born in Canada."

Trump noted that Cruz's former Canadian citizenship, which the Texas Republican renounced in 2014, has had "a huge impact on his numbers."

"I think that has a huge effect," he added. "In fact, I noticed last night, on trending, one of the biggest things on trending was, 'Can Ted Cruz run for president, based on his Canadian citizenship and his place of birth?'"

He called it "very unfair" to the Republican party if Cruz won the nomination because "the Democrats have already said they will sue."

Trump has himself pondered filing a suit against his 2016 opponent. And earlier this month, a Houston-based attorney, working independent of any campaign, officially filed a lawsuit in a U.S. District Court to clarify the "natural-born citizen" clause of the Constitution.

The Republican front-runner also slammed Cruz over failing to publicly disclose loans he took from Goldman Sachs and Citibank.

"He's supposed to be Robin Hood for everybody," Trump said. "He didn't disclose them, because he didn't want to say that he's dealing with the bankers."

When asked if Cruz's failure to file disclosure forms could have just been a mistake, Trump admitted that "it could have been."

"But," he added, "you know, it's two loans -- give me a break, okay?"

To see more of our interview with Trump, make sure you tune in on Sunday. Check your local listings for airtimes.