The two stole the show the Fox Business Network debate in North Charleston, South Carolina, where Republicans met for their second-to-last debate before the Iowa caucuses.
"I guess the bromance is over," Trump told CNN's Dana Bash afterward.
Cruz was ready to attack Trump over the real estate mogul's assertion that Cruz's Canadian birth (to a U.S. citizen mother) makes him vulnerable to accusations he is ineligible for the presidency, pointing out that Trump said in September that Cruz is, indeed, on solid legal footing.
"Since September, the Constitution hasn't changed. But the poll numbers have," Cruz said.
He said following Trump's logic to the furthest possible extent, Trump's own qualifications could be questioned due to the businessman's Scottish-born mother.
As Cruz rebutted Trump for raising the issue -- effectively winning the moment -- Trump essentially held his hands up and said he's not the one who's concerned.
"I'm beating you," Trump said. "I think I'm going to win fair and square."
And he succeeded in keeping the question alive -- a loss, in and of itself, for Cruz. Trump asked: "If you become the nominee, who the hell knows if you can even serve in office?"
That's the point Harvard law professor Laurence Tribe
, the legal source Trump has cited and Cruz has dismissed as liberal, made on CNN after the debate. "If he did put it to bed, he's certainly sleeping alone," he said.
New York, New York
Trump seemed genuinely taken aback by Cruz's insult of his "New York values."
Cruz explained that jab, saying: "Everyone understands that the values in New York City are socially liberal, are pro-abortion, are pro-gay marriage, focused around money and the media."
That led Trump to launch into a lengthy, emotional defense, pointing to New York City's response to the September 11, 2001, terror attacks.
He said that "the people of New York fought and fought and fought, and we saw more death -- the smell of death was with us for months."
"We rebuilt Downtown Manhattan, and everybody in the world watched and loved New York and loved New Yorkers," he said.
Trump's response was moving, but to Cruz it was also predictable. That Cruz stuck with his "New York values" criticism anyway highlights his strategy as the campaign enters its final days: Cruz is heavily courting the evangelicals who dominate the GOP contests in Iowa and South Carolina.
The real four-man main stage
Trump and Cruz were at the center of the night's most memorable exchanges, but Florida Sen. Marco Rubio and New Jersey Gov. Chris Christie traded blows as well.
Rubio accused Christie of supporting Common Core education standards, gun control laws and Planned Parenthood. Christie, echoing a Rubio comeback against Jeb Bush at a debate last fall, accused the senator of attacking him because he was advised it would work politically.
Later, Christie interrupted a wonky tax policy exchange between Cruz and Rubio by shifting the discussion toward entitlements -- and swatting down Rubio's attempt to interject, saying: "You already had your chance, Marco. You blew it."
That Christie was even a target suggests he has a leg up on the two other governors in the race -- Ohio Gov. John Kasich and former Florida Gov. Jeb Bush -- for the lone ticket out of the New Hampshire primary, which looks likely to winnow the "establishment" crowd.
The debate raised a question: If Bush had a good night but no one paid attention, did he really have a good night?
He hit Trump over his ban-all-Muslims proposal and trade, but Trump wouldn't talk policy and dismissed him with a line that demonstrated why he's so often gotten the best of the former Florida governor, saying: "We don't need a weak person as president of the United States, OK? ... That's essentially what we have now, and we don't need that."
Kasich, meanwhile, got the most engagement of the night from 89-year-old former Democratic Rep. John Dingell, who tweeted: "John Kasich tells more "back in my day" rocking chair stories than me or anyone I know, and I was born during the Coolidge administration."
Marco's late save
Cruz thanked Rubio for "dumping your oppo-research book on the debate stage" -- and that's exactly what Rubio had just done.
More than two hours into the debate, Rubio saved what had been a mostly lifeless night by unloading a combination of punches on Cruz.
He hit Cruz on immigration, trade and voting against Department of Defense funding -- and in doing so, he alluded to a potentially damaging theme against Cruz: That he's willing to say and do anything to win.
"That is not consistent conservatism, that is political calculation," Rubio said.
Cruz fought back, arguing that Rubio had distorted his record at every turn and saying Rubio had allied with Democrats like Sen. Chuck Schumer while he sided with Republicans like Sen. Jeff Sessions on immigration.
An NBC News/Wall Street Journal poll out Thursday showed exactly why the two are fighting. In a two-way race between Trump and Cruz, Cruz wins, 51% to 43%. But in a three-way race, Trump wins, with 40% to Cruz's 31% and Rubio's 26% -- suggesting that the two can't co-exist.
Obama's third presidential campaign
It's probably not surprising two days after the State of the Union address, but President Barack Obama was at the center of the debate.
The first question from Maria Bartiromo invited Cruz to take a shot at Obama's management of the economy -- and the Texas senator decided to also take a swipe at the President over Iranian TV pictures of the captured US naval crew on their knees with their hands on their heads.
Cruz and Christie subsequently both described the President as "a child."
There's no reason to expect the Obama talk to subside between now and November.
With his approval ratings in the mid-40s, Republicans think he's unpopular enough to be a liability for Democrats, especially when they mention him with Hillary Clinton in the same sentence. But at the same time, his high popularity among Democrats means that barring a huge slump in his favorability ratings, there's good reason for the eventual Democratic nominee not to run too far away from him either.
Undercard winner-by-default: Rand Paul
No, Kentucky Sen. Rand Paul didn't change his mind and show up for Thursday's 6 p.m. undercard round.
But skipping the debate worked for him.
Over the past couple of days, Paul appeared on CNN, Fox News and MSNBC. He was interviewed on "The Daily Show" and "The Dr. Oz Show." He held a live Twitter town hall from the social media giant's New York City offices. He Periscoped for 20 minutes during the debate. He appeared on Glenn Beck and Laura Ingraham's radio shows.
It was a lot of exposure, and he didn't miss much -- the undercard debate with Carly Fiorina, former Arkansas Gov. Mike Huckabee and former Pennsylvania Sen. Rick Santorum felt like a non-event with three candidates whose moments have come and gone.
Still, if the undercard did nothing to change those candidates' trajectories, skipping it likely did little to help Paul's. It was the best option he had, but his message with the free airtime was mostly to complain about not having been invited to the prime-time debate.
"If you're designated as someone who is not in contention, that is very disruptive to a campaign that is about three weeks out," he told CNN's Alisyn Camerota on "New Day" Thursday. "There's only one debate tonight. Let's be honest about this."