While there has been plenty of animosity between Trump and most of his rivals, the billionaire businessman and Cruz have been on largely friendly territory for much of the campaign season. That's changed, however, as the polls in Iowa tighten ahead of the February 1 caucuses.
Trump has repeatedly questioned Cruz' eligibility to become president, seeking to eat into the senator's support in Iowa. Cruz insists he meets the constitutional requirements to serve because he's a "natural-born" citizen with an American mother.
But in his attack against Trump on Thursday, Cruz noted that some of the more extreme theories on the topic conclude that someone can only become president if both parents were born in the United States. Under that standard, Cruz noted, Trump himself would be ineligible for the presidency because his mother was born in Scotland."the US
"On the issue of citizenship, Donald, I'm not going to use your mother's birth against you," Cruz said at the debate sponsored by the Fox Business Network.
Trump shot back: "Because it wouldn't work."
In one of the most memorable exchanges of the night, Trump responded to Cruz' recent accusation that the businessman "embodies New York values."
Asked to explain the meaning behind the insult, Cruz said New Yorkers tend to hold "socially liberal" views, and its residents tend to revolve around "money and media."
"Not a lot of conservatives come out of Manhattan. I'm just saying," he said.
Trump appeared more than prepared with a response. He recalled seeing the towers of the World Trade Center collapse on September 11, 2001. The resilience of New Yorkers in the aftermath and the rebuilding of the city that followed, Trump argued, revealed the true spirit of New York.
"I saw something that no place on Earth could have handled more beautifully, more humanely than New York," Trump said. "The people in New York fought and fought and fought. We saw more death and even the smell of death and it was with us for months."
Having clearly won the moment, Trump added: "We rebuilt downtown Manhattan and everybody in the world watched and everybody in the world loved New York and loved New Yorkers. And I have to tell you, that was a very insulting statement that Ted made."
"Here you have Tump leading by such a wide margin over other people," says Dr. David Kozak, Political Science Professor at Gannon University. "But, not a single prominent member of the US House or Senate or not a single Governor has yet to endorse him. We normally have a whole series of endorsements by this time."
Cruz also had sharp words for The New York Times for publishing a story this week that said Cruz failed to properly disclose large loans he received during his Senate campaign.
Asked about the controversy, Cruz quickly retorted: "Thank you for passing on that hit piece on the front page of The New York Times."
"The entire New York Times attack is that I disclosed that loan on one filing with the United States Senate ... but it was not in the second filing with the FEC," Cruz said. "Yes, I made paperwork errors ... but if that's the best hit the New York Times has got, they better go back to the well."