The 2.5-hour event sponsored by Fox Business Network was filled with testy exchanges between the seven candidates on stage. Cruz and Trump are battling for first place in Iowa with less than three weeks until the state's caucuses, though the businessman has a commanding lead nationally. And with pressure mounting for someone to emerge as an establishment alternative to Trump and Cruz, sparks flew between Marco Rubio and Chris Christie.
The much-anticipated Trump vs. Cruz showdown took a few minutes to materialize — but when it did, it packed a punch.
Cruz forcefully responded to Trump's accusations that he isn't eligible to be president because he was born in Canada -- a controversy that Trump has only recently embraced.
"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, hoping to eat into the senator's support in Iowa, has repeatedly questioned whether Cruz, whose mother was a U.S. citizen, is a natural born citizen.
But in his attack against Trump on Thursday, Cruz noted that some of the more extreme theories on the topic would 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.
Trump shot back: "Because it wouldn't work."
It wasn't just Trump who put Cruz on the defensive. In the final moments of the debate, Rubio slammed his fellow senator for hiding behind the pretense of conservative values.
In one of his strongest moments of the evening, Rubio listed a string of issues on which he said Cruz had flip-flopped, including immigration and ethanol subsidies.
"That is not consistent conservatism," he says. "That is political calculation."
When Cruz shot back, "I appreciate you dumping your oppo research file on the debate stage," Rubio was ready with a sharp retort: "It's your record."
'New York values'
Trump was more dominant Thursday than in previous debates. 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 what that meant, 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," he said.
Trump was ready with an emotional response. He recalled seeing the World Trade Center towers collapse on September 11, 2001. The resilience of New Yorkers in the aftermath, Trump said, 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
Christie and Rubio also tussled on stage, as the two men touted their conservative bona fides. 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 responded, pointing to an infamous moment in a past debate when Rubio accused Bush of attacking him only because someone had instructed the former Florida governor 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."
Muslim ban, gun control
The candidates also sparred over Trump's controversial proposal to temporarily ban Muslims from entering the U.S. The idea triggered waves of criticism 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 build 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 Syrian refugees into the country.
"I've said from the beginning, we should take no Syrian refugees of any kind," Christie said.
And coming on the heels of Obama's new executive actions on gun control, several candidates offered impassioned defenses of Second Amendment rights.
"I am convinced that if this president could confiscate every gun in America, he would," Rubio said.
Christie, meanwhile, blasted the actions as inconsistent with democracy.
"That's not a democracy, that's a dictatorship," he said.
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 turned to Trump.
"Despite Donald Trump's bromance with Vladimir Putin ... Russia is our adversary," she said.