Crowd boos Donald Trump during epic showdown with Ted Cruz over Canadian birth issue
US Sen. Ted Cruz (R-Texas) and real-estate tycoon Donald Trump sparred extensively during the early portion of Thursday night's Republican debate over Cruz's birth in Canada.
Trump has brought up questions about Cruz's eligibility to run for president on a recurring basis in recent weeks, as Cruz has surged in polls of the first-caucus state of Iowa.
When asked about the issue by Fox moderator Neil Cavuto, Cruz accused "my friend Donald" of only questioning is eligibility for president because of that fact — and because the real-estate mogul's poll numbers have been sinking.
Click through images from the first 2016 GOP presidential debate:
"Back in September, my friend Donald said he had his lawyer look at this from every which way and there was no issue there. There was nothing to this 'birther' issue. Since September, the Constitution hasn't changed. But the poll numbers have," Cruz said to whistles and cheers from the crowd.
"And I recognize that Donald is dismayed that his poll numbers are falling in Iowa," he added. "But the facts and the law here are really quite clear."
SEE ALSO: RECAP: The big Republican debate
Cruz then explained why many legal scholars think he is clearly eligible to be president: Because the Texas senator's mother was American at the time of his birth. Cruz claimed that some of the more "extreme" theories on the internet argue that a potential president must have two US parents in order to qualify for the Oval Office.
"On the issue of citizenship, Donald, I'm not going to use your mother's birth against you," he quipped.
Trump responded by touting his poll numbers. But he was booed while citing a new NBC News/Wall Street Journal poll, published earlier on Thursday, that gave him a 13-point national lead over Cruz. The businessman claimed that the crowd was booing Cruz, not him.
"You have a big lawsuit over your head while you're running. And if you become the nominee, who the hell knows if you can even serve in office," Trump said.
SEE ALSO: Carly Fiorina: Unlike Hillary Clinton, 'I actually love spending time with my husband'
Cruz, a former Texas solicitor general who has argued cases before the US Supreme Court, dismissed Trump's ability to argue legal issues with him.
"I've spent my entire life defending the Constitution before the US Supreme Court. And I'll tell you, I'm not going to be taking legal advice from Donald Trump," Cruz quipped.
He also earned laughter by joking to offer to name Trump as his vice-presidential nominee. Cruz joked that if he were disqualified from the presidency, Trump then could take over at the top of the ticket.
More on AOL.com:
Rand Paul literally flips off media after debate firestorm
Planned Parenthood files US lawsuit against group behind secret videos
Loads of Americans are moving out of these 18 states
