Republican presidential bets Donald Trump and Ted Cruz swapped verbal jabs as the five other top candidates contented themselves watching the spectacle on Thursday in the first GOP presidential debate this year.

Trump and Cruz slugged it out with less than three weeks before the first votes of the 2016 U.S. election season and with Trump holding a 13-point lead over Cruz in the latest polls.

Although the latest NBC News-Wall Street Journal poll showed Trump more than doubling his lead over Cruz, the same poll indicated that Cruz would beat Trump by eight points in a hypothetical contest between the two of them.

The national survey of 400 GOP primary voters showed Trump with 33 percent support; Cruz, 20 percent; Marco Rubio, 13 percent; Ben Carson, 12 percent; Chris Christie and Jeb Bush, 5 percent each; Carly Fiorina, John Kasich and Rand Paul, 3 percent each; and Mike Huckabee, 1 percent.

Trump's 13 percent lead over Cruz is more than double his 5 percent lead in a December survey, according to Newsmax.

In a hypothetical match-up between Trump and Cruz, the results are: Cruz, 51 percent; Trump, 43 percent.

In a match-up between Trump and Rubio, the results are: Trump, 52 percent; Rubio, 45 percent.

The NBC News-Wall Street Journal poll was conducted between Saturday and Wednesday, with a margin of error of 4.9 percent.

In Thursday night's debate in North Charleston, South Carolina, ahead of the Iowa caucuses on Feb. 1 and hosted by Fox Business Network, Trump and Cruz clashed mainly on the issue of election eligibility, WND reported.

Trump is questioning Cruz' eligibility to run for president since the Texas senator was born in Alberta, Canada, to an America mother and Cuban father, even though Cruz gave up any claim to Canadian citizenship in 2014.

Cruz defended himself, saying, "Under longstanding U.S. law, the child of a U.S. citizen born abroad is a natural-born citizen. If a soldier has a child abroad, that child is a natural-born citizen. That's why John McCain, even though he was born in Panama, was eligible to run for president."

Trump insisted that the issue of Cruz's Canadian birth is a "serious question," saying that some "very, very fine constitutional attorneys feel that because he was not born on the land he cannot run for office."

Cruz countered, saying that Trump would also not be eligible for the presidency under his own interpretation of the Constitution, noting that Trump's mother was born in Scotland.

Trump butted in, "But I was born here. Big difference."

Cruz attacked Trump, accusing him of not being a true conservative since he comes from New York and represented "New York values."

"Everyone understands that the values in New York City are socially liberal, are pro-abortion or pro-gay marriage. Focus around money and the media," the Texas senator said.

Trump slammed Cruz for insulting the people of New York. He then recalled the terrorist attack in New York on Sept. 11, 2001 when the World Trade Center came crashing down after being hit by two terrorist-commandeered airliners.

"When the World Trade Center came down, I saw something that no place on Earth could have handled more beautifully, more humanely than New York," Trump said.

That line drew applause from the candidates, including Cruz.

"You had two 110-story buildings come crashing down. I saw them come down. And the cleanup started the next day and it was the most horrific cleanup probably in the history of doing this, and in construction. I was down there. And I've never seen anything like it. And the people in New York fought and fought and fought. We saw more death and even the smell of death. Nobody understood it. And it was with us for months. The smell was in the air. And everybody in the world watched and everybody in the world loved New York and loved New Yorkers," Trump said.

The candidates, as expected, took potshots at President Barack Obama and Democratic presidential front-runner Hillary Clinton.

Cruz blasted Obama for his appeasement policy on Iran.

"Today, many of us picked up our newspapers and were horrified to the see the sight of 10 American sailors on their knees with their hands on their heads," he said. "In that State of the Union (address) President Obama didn't so much as mention the 10 sailors that had been captured by Iran."

Dismissing this humiliating incident, Obama is now "preparing to send $100 billion, or more, to Ayatollah Khameini. And, I'll tell you, it was heartbreaking," Cruz said.

Bush aimed his tirade at Clinton, saying she "would be a national security disaster."

Rubio then added, "I would go one step further in this description of Hillary Clinton. She wouldn't be just a disaster."