As Texas Sen. Ted Cruz's stock continues to rise, so have the attacks from his rivals.

Case and point: Thursday night's GOP presidential debate on Fox Business Network, where both businessman Donald Trump and Florida Sen. Marco Rubio went after their fellow Republican candidate on his place of birth, stance on immigration and voting record, among other topics.

Early on, Cruz was forced to defend his ability to serve as president after Trump raised questioned about the Texas lawmakers eligibility to run for the office given that he was born in Canada. The senator went on to accuse Trump of raising the issue only because polls show Cruz is challenging the billionaire businessman's lead, particularly in Iowa.

"The Constitution hasn't changed — but the poll numbers have," Cruz said. The senator was born in Canada, but his mother is American, which legal scholars agree fits with the Constitution's provision that only a "natural born citizen" may be president.

Trump didn't dispute that he's brought up the issue of Ted Cruz' citizenship because Cruz "is doing a little better. It's true."

And he said he doesn't necessarily believe Cruz isn't eligible. He says only that Democrats will challenge Cruz in court should he become the Republican nominee.

"There's a big question mark on your head and you can't do that to the party," Trump said. "You have to have certainty." He is urging Cruz to ask courts for a declaratory judgment to settle the matter."

Cruz cited the widely accepted legal principle that anyone born to an American parent is a natural-born citizen, regardless of where the child is born. He says Trump is basically claiming that a natural-born citizen would have to be born in the United States to two parents who were also born in the United States.

That standard, Cruz said would disqualify several candidates. Among them: Donald Trump, whose mother was born in Europe.

"You're an American, as is everybody else on this stage," Cruz said. "I would suggest we focus on who is best prepared to be commander-in-chief."

But it wasn't just Trump attacking Cruz.

Late in the debate, Rubio and Cruz reignited their fight from previous debates over immigration, with Cruz bashing Rubio for supporting the so-called "gang of eight" bipartisan immigration reform law. The Florida lawmaker then slammed his rival for allegedly flip-flopping on the issue.

"That's not consistent conservatism," Rubio said. "That's political maneuvering."

Thursday's debate comes less than three weeks before the Iowa caucuses kick off this year's voting. Trump has led the Republican field for months, confounding GOP leaders and many of his rivals.

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

When Cruz was asked about a comment he made days earlier when he said the problem with Trump as his "New York values," Trump quickly chimed in.

"That was a very insulting statement that Ted made," Trump added.

Trump said he was insulted. He invoked Sept. 11 and the way New Yorkers came together after the attacks to deal with "the smell, the air."

The on-stage arguments between the two candidates signaled an end to months of relative civility between Trump and Cruz, both of whom are appealing to Republican voters deeply frustrated with Democrats in Washington and sometimes with their own party leaders.

The more mainstream candidates on stage fought to edge their way into the debate.

"I hate to interrupt this episode of Court TV," Rubio said amid the wrangling between Trump and Cruz.

While battles with Cruz took center stage during Thursday's debate, other candidates also had their time to take shots at others on stage.

Rubio was hit by New Jersey Gov. Chris Christie, who said the lawmaker talks so much it's hard to keep track of whether he's telling the truth.

Christie's comment came after Rubio first went after Christie, accusing him of backing Planned Parenthood, Common Core academic standards and gun control.

"We can't have a president of the United States that supports gun control," Rubio said.

Christie said that two years ago Rubio called him a "conservative reformer" but now he is attacking him and misrepresenting his record. Christie says he has not supported Planned Parenthood and he has taken action as governor to protect 2nd Amendment gun rights.

Christie says, "When you're a senator what you get to do is talk and talk and talk and no one can keep up to see if what you're saying is accurate or not." He says governors are held accountable.

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