North Charleston, South Carolina (CNN)Donald Trump and Ted Cruz clashed Thursday in their sharpest -- and most personal -- encounters of the campaign season.

"I guess the bromance is over," Trump told CNN's Dana Bash after the debate.

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.

Fox Business GOP debate in 90 seconds 01:30

"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."

READ: 6 takeaways from the debate

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.