 -- With two weeks left before voters in Iowa cast the very first votes in the 2016 presidential election, Republican front-runner Donald Trump is turning up the heat on his fiercest rival in the Hawkeye State -– Sen. Ted Cruz.

"I don't think Ted Cruz has a great chance, to be honest with you," Trump told ABC News Chief Anchor George Stephanopoulos in an interview on “This Week” Sunday. "Look, the truth is, he's a nasty guy. He was so nice to me. I mean, I knew it. I was watching. I kept saying, 'Come on Ted. Let's go, okay.' But he's a nasty guy. Nobody likes him. Nobody in Congress likes him. Nobody likes him anywhere once they get to know him. He's a very –- he's got an edge that's not good. You can't make deals with people like that and it's not a good thing. It's not a good thing for the country. Very nasty guy."

On the campaign trail, Trump is now questioning the freshmen Texas senator for a loan, first reported by The New York Times, that Cruz took out from Goldman Sachs during his 2012 Senate run that he failed to disclose in federal campaign finance documents.

"He's a total hypocrite. How about his fundraising and how about when he does his personal financial disclosure form, and he doesn't put on that he's borrowing money from Goldman Sachs? And then today it comes out that he's also borrowing money from CitiBank and he doesn't list it." Trump told Stephanopoulos. "You know why? He wants to look like Robin Hood that he's the one protecting the people from the banks while he's actually borrowing money and personally guaranteeing it and not disclosing it, which is illegal."

Campaigning Saturday afternoon in South Carolina, Cruz did not hold back when it came to his "friend" Donald Trump.

"It seems Donald has a lot of nervous energy –- it seems Donald doesn't react well when he's going down in the polls," Cruz said. "You know in terms of a commander in chief, I think we ought to have someone who isn't springing out of bed and tweeting in response to the latest polls. Stable and steady and a calm hand to keep this country safe." Trump sent a series of tweets aimed at Cruz early Saturday morning.

In an interview with The Washington Post earlier this month, Trump was asked if Cruz was eligible to run for the presidency, citing the fact he was born in Canada. In September, Trump had told ABC News’ Jonathan Karl that he felt Cruz, who was born to an American mother, was in good standing. But in recent weeks, including at Thursday's GOP presidential debate, he has said he fears a lawsuit on his eligibility will prevent Cruz from running.

Stephanopoulos told Trump that some legal scholars have suggested Trump himself would have standing to sue Cruz.

"Oh, that's an interesting case. Wow, that sounds like a very good case. I'd do the public a big favor," Trump responded, but he would not say whether he'd actually file such a suit. "It's a good idea-- maybe I'll talk to them about it. I'd like to talk to Ted about it, see how he'd feel about it. 'Cause you know, when I file suits, I file real suits."

ABC News' Jessica Hopper contributed to this report.