But in Thursday night's debate, Trump admitted that he was bringing up the issue because Cruz has been "doing a little bit better" in the polls. Most legal scholars admit that the question of whether a person who was born outside of the United States to an American citizen qualifies as a "natural-born citizen", as defined by the Constitution.

The Republican presidential candidates slammed the mainstream media during a debate on Thursday, repeatedly singling out the New York Times for criticism to the delight of the audience.

Republican presidential candidate, businessman Donald Trump, left, speaks as Republican presidential candidate, Sen.

In addition to fighting back over his presidential eligibility, Cruz accused Trump of not being a conservative because he was born in NY and still lives there.

In an interview aired on CNN after the debate, Trump said: "You can't insult New Yorkers in front of me".

When Neil Cavuto asked Ted Cruz about his birth, Cruz quoted Trump from September. Ted Cruz's eligibility to serve as president, telling his rival his birth in Canada leaves "a big question mark on your head". You have to have certainty, you can't have a question.

Trump said Democrats would sue if Cruz were on the Republican ticket, putting their party's chances of winning at risk.

This time, it was Christie responding to Rubio's attacks: "I stood on the stage and watched Marco in rather indignantly, look at Governor Bush and say, someone told you that because we're running for the same office, that criticizing me will get you to that office. 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.

Cruz countered that Trump was simply invoking the recent punditry of Laurence Tribe, a Harvard Law School professor whose former students include Cruz and Barack Obama (to whom Tribe was a mentor).

But in a second exchange, Cruz was asked to explain what his campaign trail jab at Trump's "New York values" meant.

The president gave his final State of the Union address on Tuesday and offered an optimistic vision of America's future as a contrast to the Republican field of candidates trying to replace him.

Rubio likened Christie's policies to President Obama's, particularly on guns and education reform - an attack Christie declared false. "Only the U.S. Supreme Court can finally decide, determine judicially and settle this issue now".

Marco Rubio - Rubio is the most obvious beneficiary if Trump and Cruz continue to battle each other. In his closing statement, Cruz mentioned the Michael Bay movie "13 Hours" which depicts the attack on a USA diplomatic compound in Benghazi, Libya, that occurred during Clinton's tenure as secretary of state.

Rubio will likely take fire for his role in the 2013 immigration debate, while even a strong performance from Bush may not be enough to allay concerns that his campaign is at risk of faltering. Gov. Chris Christie of New Jersey also tried to stay above the fray, deriding Ms. Clinton as an Obama carbon copy. Sen.

Christie adds that a Hillary Clinton presidency will be a "third term of Barack Obama's leadership".

The annual deficit actually has been cut by more than half during the seven years of the Obama administration.

"Everybody's record's going to be scrutinized, and at the end of the day, we need to be united behind the victor so we can defeat Hillary Clinton, because she is a disaster", he said.

CRUZ: You're from NY, so you might not.

But a good portion of the debate pitted the number one and the number two candidates in the polls - Donald Trump and Sen.

"The rest of the country knows what NY values are", Cruz said on Fox News. A lawsuit against Cruz's candidacy has already been launched, and more are expected.

Trump, who had maintained a cordial stance with Cruz for month, began unleashing scathing attacks on the Texas senator during the debate.