Crowd boos Donald Trump during epic showdown with Ted Cruz over Canadian birth issue

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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.

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Crowd boos Donald Trump during epic showdown with Ted Cruz over Canadian birth issue
Republican Presidential candidate businessman Donald Trump looks on during the Republican Presidential debate sponsored by Fox Business and the Republican National Committee at the North Charleston Coliseum and Performing Arts Center in Charleston, South Carolina on January 14, 2016. AFP PHOTO/ TIMOTHY A. CLARY / AFP / TIMOTHY A. CLARY (Photo credit should read TIMOTHY A. CLARY/AFP/Getty Images)
Republican Presidential candidates, businessman Donald Trump (R) and Florida Senator Marco Rubio shake hands after the Republican Presidential debate sponsored by Fox Business and the Republican National Committee at the North Charleston Coliseum and Performing Arts Center in Charleston, South Carolina on January 14, 2016. AFP PHOTO/ TIMOTHY A. CLARY / AFP / TIMOTHY A. CLARY (Photo credit should read TIMOTHY A. CLARY/AFP/Getty Images)
Republican presidential candidate, New Jersey Gov. Chris Christie, left, speaks as Republican presidential candidate, Sen. Marco Rubio, R-Fla., takes notes during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
NORTH CHARLESTON, SC - JANUARY 14: Republican presidential candidates (L-R) Donald Trump and Sen. Ted Cruz (R-TX) speak during a commercial break in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
NORTH CHARLESTON, SC - JANUARY 14: Republican presidential candidate Ben Carson participates in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
Republican presidential candidate, Sen. Ted Cruz, R-Texas, speaks during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Rainier Ehrhardt)
Republican presidential candidate, Sen. Ted Cruz, R-Texas, holds his daughter at a break during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, businessman Donald Trump, left, speaks as Republican presidential candidate, Sen. Ted Cruz, R-Texas, speaks during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Rainier Ehrhardt)
NORTH CHARLESTON, SC - JANUARY 14: Audience members listen to the Republican presidential candidates participate in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
NORTH CHARLESTON, SC - JANUARY 14: Republican presidential candidate Ohio Governor John Kasich participates in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
NORTH CHARLESTON, SC - JANUARY 14: Republican presidential candidates (L-R) Donald Trump, Sen. Ted Cruz (R-TX) and Ben Carson participate in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
Republican presidential candidate, Sen. Ted Cruz, R-Texas, speaks during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, former Florida Gov. Jeb Bush speaks during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican Presidential candidate businessman Donald Trump speaks during the Republican Presidential debate sponsored by Fox Business and the Republican National Committee at the North Charleston Coliseum and Performing Arts Center in Charleston, South Carolina on January 14, 2016. AFP PHOTO/ TIMOTHY A. CLARY / AFP / TIMOTHY A. CLARY (Photo credit should read TIMOTHY A. CLARY/AFP/Getty Images)
NORTH CHARLESTON, SC - JANUARY 14: Republican presidential candidates (L-R) Sen. Marco Rubio (R-FL), Donald Trump and Sen. Ted Cruz (R-TX) participate in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
NORTH CHARLESTON, SC - JANUARY 14: Republican presidential candidate Sen. Marco Rubio (R-FL) participates in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
NORTH CHARLESTON, SC - JANUARY 14: Republican presidential candidates (L-R) Ohio Governor John Kasich, New Jersey Governor Chris Christie, Sen. Marco Rubio (R-FL), Donald Trump, Sen. Ted Cruz (R-TX), Ben Carson and Jeb Bush participate in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
NORTH CHARLESTON, SC - JANUARY 14: Republican presidential candidates (L-R) Donald Trump, Sen. Ted Cruz (R-TX) and Ben Carson participate in the Fox Business Network Republican presidential debate at the North Charleston Coliseum and Performing Arts Center on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
Republican presidential candidate, businessman Donald Trump speaks as Republican presidential candidate, Sen. Ted Cruz, R-Texas, looks on during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, Sen. Ted Cruz, R-Texas, speaks during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, Sen. Marco Rubio, R-Fla., speaks during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, New Jersey Gov. Chris Christie speaks during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, retired neurosurgeon Ben Carson waves during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, Sen. Marco Rubio, R-Fla., left, speaks as Republican presidential candidate, businessman Donald Trump looks on during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Rainier Ehrhardt)
Republican presidential candidate, retired neurosurgeon Ben Carson laughs during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, Sen. Ted Cruz, R-Texas, waves during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, Sen. Marco Rubio, R-Fla., speaks as Republican presidential candidate, businessman Donald Trump looks on during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Rainier Ehrhardt)
Republican presidential candidates, from left, Ohio Gov. John Kasich, New Jersey Gov. Chris Christie, Sen. Marco Rubio, R-Fla., businessman Donald Trump, Sen. Ted Cruz, R-Texas, retired neurosurgeon Ben Carson and former Florida Gov. Jeb Bush take the stage before the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Rainier Ehrhardt)
Republican presidential candidate, former Arkansas Gov. Mike Huckabee speaks as Republican presidential candidate, businesswoman Carly Fiorina looks on during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Rainier Ehrhardt)
Republican presidential candidate, former Pennsylvania Sen. Rick Santorum speaks during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, former Arkansas Gov. Mike Huckabee leaves the stage after the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Rainier Ehrhardt)
Republican presidential candidate, former Pennsylvania Sen. Rick Santorum speaks during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Rainier Ehrhardt)
Republican presidential candidate, former Florida Gov. Jeb Bush waves to the audience during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Rainier Ehrhardt)
NORTH CHARLESTON, SC - JANUARY 14: (L-R) Neil Cavuto and Maria Bartiromo, moderators of the Fox Business Network Republican presidential debate speak to the audience on stage at the North Charleston Coliseum and Performing Arts Center prior to the debate on January 14, 2016 in North Charleston, South Carolina. The sixth Republican debate is held in two parts, one main debate for the top seven candidates, and another for three other candidates lower in the current polls. (Photo by Scott Olson/Getty Images)
Republican presidential candidate, businessman Donald Trump stands on the stage before the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
Republican presidential candidate, businessman Donald Trump stands on the stage during the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
South Carolina Gov. Nikki Haley, right, views the candidates before the Fox Business Network Republican presidential debate at the North Charleston Coliseum, Thursday, Jan. 14, 2016, in North Charleston, S.C. (AP Photo/Chuck Burton)
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"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.

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