GOP debate: The Trump-Cruz smackdown

NORTH CHARLESTON, South Carolina (CNN) -- The much-anticipated Donald Trump vs. Ted Cruz showdown took a few minutes to materialize at the Republican presidential debate here --- but when it did, it packed a punch.

Cruz forcefully responded to Trump's accusations that he's not eligible to be president because he was born in Canada.

"Back in September, my friend Donald said he had his lawyers look at this in every which way," Cruz said. "There was nothing to this birther issue."

He added: "Since September, the Constitution hasn't changed. But the poll numbers have."

While there has been plenty of animosity between Trump and most of his rivals, the billionaire businessman and Cruz have been on largely friendly territory for much of the campaign season. That's changed, however, as the polls in Iowa tighten ahead of the February 1 caucuses.

Trump has repeatedly questioned Cruz' eligibility to become president, seeking to eat into the senator's support in Iowa. Cruz insists he meets the constitutional requirements to serve because he's a "natural-born" citizen with an American mother.

But in his attack against Trump on Thursday, Cruz noted that some of the more extreme theories on the topic conclude that someone can only become president if both parents were born in the United States. Under that standard, Cruz noted, Trump himself would be ineligible for the presidency because his mother was born in Scotland."

"On the issue of citizenship, Donald, I'm not going to use your mother's birth against you," Cruz said at the debate sponsored by the Fox Business Network.

Trump shot back: "Because it wouldn't work."

'New York values'

In one of the most memorable exchanges of the night, Trump responded to Cruz' recent accusation that the businessman "embodies New York values."

Asked to explain the meaning behind the insult, Cruz said New Yorkers tend to hold "socially liberal" views, and its residents tend to revolve around "money and media."

"Not a lot of conservatives come out of Manhattan. I'm just saying," he said.

Trump appeared more than prepared with a response. He recalled seeing the towers of the World Trade Center collapse on September 11, 2001. The resilience of New Yorkers in the aftermath and the rebuilding of the city that followed, Trump argued, revealed the true spirit of New York.

"I saw something that no place on Earth could have handled more beautifully, more humanely than New York," Trump said. "The people in New York fought and fought and fought. We saw more death and even the smell of death and it was with us for months."

Having clearly won the moment, Trump added: "We rebuilt downtown Manhattan and everybody in the world watched and everybody in the world loved New York and loved New Yorkers. And I have to tell you, that was a very insulting statement that Ted made."

Cruz also had sharp words for The New York Times for publishing a story this week that said Cruz failed to properly disclose large loans he received during his Senate campaign.

Asked about the controversy, Cruz quickly retorted: "Thank you for passing on that hit piece on the front page of The New York Times."

"The entire New York Times attack is that I disclosed that loan on one filing with the United States Senate ... but it was not in the second filing with the FEC," Cruz said. "Yes, I made paperwork errors ... but if that's the best hit the New York Times has got, they better go back to the well."

Alternatives to Trump and Cruz

Cruz and Trump were joined on the prime-time stage by Marco Rubio, Jeb Bush, Chris Christie and John Kasich. Pressure is mounting for someone from the rest of the field to emerge as the clear alternative to Trump and Cruz.

Christie and Rubio engaged in a tough exchange as they both try to dominate the establishment lane of the party. Rubio accused Christie of being on the same page as President Barack Obama on issues such as gun control, Common Core and support for Planned Parenthood.

"Unfortunately, Gov. Christie has endorsed many of the ideas that Barack Obama supports," the Florida senator said.

Christie deftly fired back, pointing to a memorable moment in a past debate when Rubio accused Bush of attacking Rubio only because someone had instructed him to do so.

It appears that "same someone has been whispering in old Marco's ear, too," Christie said.

He pushed back on Rubio's accusations as he touted his executive experience in the New Jersey governor's mansion.

"This is the difference between being a governor and being a senator ... when you're a senator, what you get to do is just talk and talk and talk," Christie said. "Two years ago, he called me a conservative reformer that New Jersey needed. That was before he was running against me."

The candidates also took plenty of shots at Obama. Asked about the state of the economy, Cruz instead answered by criticizing Obama for not mentioning in his State of the Union address the 10 U.S. sailors who were captured by Iran earlier this week.

"Today, many of us picked up our newspapers and we were horrified to see the sight of 10 American sailors on their knees," Cruz said. When he is president, he added, "No serviceman or any woman will be forced to be on their knees."

Bush, meanwhile, said the idea that the country is better off today than before Obama took office is an "alternative universe." Pointing to the growing threat of ISIS -- and slamming Obama for referring to the group as the "JV team," Bush said the President was "missing the whole point" on America's role in the world.

"For the life of me, I have no understanding why the President thinks everything is going on," he said.

And Rubio blasted Obama for presiding over an "arrogant" foreign policy.

Muslim ban

The candidates also sparred over Trump's controversial proposal to temporarily ban Muslims from entering the U.S. The idea has set off alarms within the Muslim-American community, but many of Trump's supporters say it holds merit in light of the growing threat of ISIS.

"We have to stop with political correctness," Trump said, insisting he would not change his mind on the issue.

Bush repeatedly called on Trump to "reconsider," warning that the idea makes it "impossible" to built a coalition with other Arab countries necessary to defeat ISIS.

"All Muslims? Seriously? What kind of signal does that send to the rest of the world?" Bush said.

Others, including Kasich and Christie, said they were on board with at least temporarily not allowing in Syrian refugees into the country.

"I've said from the beginning, we should take no Syrian refugees of any kind," Christie said.

Undercard debate

Earlier Thursday, Carly Fiorina dominated a smaller, less-glamorous debate stage when she joined two other low-polling candidates to discuss everything from national security to technology and Hillary Clinton.

Fiorina's candidacy has largely been defined by memorable debate performances. And even though she was dropped from the prime-time stage at the debate, she still delivered. Right out of the gate, she dealt a sharp personal attack on Clinton, the Democratic front-runner.

"I'm not a political insider. I haven't spent my lifetime running for office," the former Hewlett-Packard CEO said. "And unlike another woman in this race, I actually love spending time with my husband."

Minutes later, Fiorina trained her fire on Trump.

"Despite Donald Trump's bromance with Vladimir Putin ... Russia is our adversary," she said.

Mike Huckabee and Rick Santorum joined Fiorina on the stage. All three candidates were sharply critical of Obama on issues such as gun control, national security and the economy.

"We have a president who seems to be more interested in protecting the reputation and image of Islam than protecting us," Huckabee said.

Santorum, whose core base includes blue-collar conservatives, slammed the President on job creation.

"The numbers just don't add up," he said. Obama "has done more to take jobs away from the hard-working people who are struggling the most."

Rand Paul, who also didn't qualify for the main debate, skipped the event.

"People have to realize that what the media is doing here is predeciding an election," Paul said on CNN's "New Day."