The New York Times reported this week that Cruz had failed to disclose nearly a million dollars in loans from Goldman Sachs and Citibank in his FEC Senate filing.
Cruz seemed prepared and confident as he addressed the questions.
"Thank you for passing on that hit piece on the front page of The New York Times," Cruz told the debate's moderators.
Cruz has called it an "inadvertent" mistake and said his campaign has reached out to the FEC to amend the filing if needed.
"We took a loan against our assets to invest it in that campaign to defend ourselves against those attacks and the entire New York Times attack is that I disclosed that loan on one filing with the United States Senate," he said. "That was a public filing, but it was not on a second filing with the FEC. Both of those filings were public. And yes, I made a paperwork error disclosing it on one piece of paper instead of the other. But if that is the best hit "New York Times" has got, they better go back to the well."
"Back in September, my friend Donald said he had his lawyers look at this from every which way and there was no issue. There was nothing to this birther issue. Now since September, the Constitution hasn't changed, but the poll numbers have," Cruz said. "And I recognize...that Donald is dismayed that his poll numbers are falling in Iowa. But the facts and law here are really quite clear."
Earlier this week, Trump said that Cruz should get a declaratory judgment that he is a natural born citizen to avoid Democrats challenging his eligibility. Trump has also relied on the opinion of Harvard Law Professor Lawrence Tribe who has questioned Cruz's natural born citizenship. Cruz used his answer to describe Tribe as a Democratic "left-wing judicial activist" and said that Democrats were echoing Trump's criticisms because they'd rather run against Trump than Cruz.
Cruz used his legal expertise as a past solicitor general of Texas to debunk worries about his citizenship.
"Listen, I spent my entire life defending the Constitution before the U.S. Supreme Court and I'll tell you I'm not going to be taking legal advice from Donald Trump," Cruz said. "Well, I’ll tell you also is the chances of any litigation proceeding and succeeding on this, are zero."
Through the course of Cruz's answer, Trump interjected to explain why he raised the questions about Cruz's eligibility.
"I choose him as my vice presidential candidate and if the Democrats sue, because we can't take him along for the ride. I don't like that. Okay?" Trump said.
Using the humor he often infuses into his campaign stump speeches, Cruz countered.
"And I'll tell you what, Donald, you very kindly just a moment ago offered me the VP slot," Cruz said. "I'll tell you what. If this all works out, I'm happy to consider naming you as VP and so if you happen to be right, you could get the top job at the end of the day."
"I think I will go back to building buildings if it doesn't work out," Trump said.