Rubio attacked Trump's character. And Ted Cruz followed up by questioning Trump's conservative credentials.
The big questions of the night: Which senator did a better job convincing voters they can best take on Trump? Did Rubio's attacks and interruptions show a new side of himself? Did Cruz do enough to persuade people he's worth another look? Or did Trump's dismissive counters -- Rubio is a "choke artist," while Cruz is a "liar" -- leave him looking like a strongman swatting away the sorts of politicians that turned his supporters furious in the first place?
From the opening minutes, Rubio mercilessly prodded, slammed and taunted Trump, talking over him in the sort of sustained way that Jeb Bush never could.
Rubio called the real estate mogul's Trump University "a fake school." He invoked Trump's business record to question his sincerity on immigration, saying: "You're the only person on this stage that's ever been fined for hiring people that worked on your projects illegally."
When Trump dismissed those allegations as old news, Rubio shot back: "I guess there's a statute of limitations on lies."
Later, as Trump insisted that the crux of his health care plan would involve allowing insurance purchases across state lines, Rubio pressed for more specifics, saying that "now he's repeating himself" -- an ironic response from a candidate who has been mocked as robotic for repeating talking points at speeches and debates.
"I don't repeat myself," Trump said.
"He repeats himself every day," Rubio answered, adding that Trump's refrains are all familiar: "Everybody's dumb, we're gonna make America great again, we're gonna win, win, win..."
Rubio also got in a memorable retort on Israel. Even as Trump called himself "totally pro-Israel," he said he didn't believe there was any reason for labeling Israel and the Palestinians as the "good guy" and the "bad guy."
"The position you've taken is an anti-Israel position," Rubio said.
When Trump said he was simply a "negotiator," Rubio shot back: "The Palestinians are not a real estate deal, Donald."
Through it all, Rubio kept a smile on his face -- almost as if to say to the audience, "Can you believe this guy?"
Why Rubio was relentless against Trump
For Rubio, it was now or never
. His attacks, and those of Cruz, weren't necessarily new in substance -- Trump isn't a conservative, Trump is untrustworthy, and so on -- but what stood out was Rubio's sense of urgency to put himself center stage with the billionaire front-runner.
What it was about: Lighting a fire under the donor class and GOP establishment.
Rubio has to demonstrate that he's worth a massive investment -- right this minute -- to try to block Trump from winning a nomination that the establishment grows more convinced by the day is his for the taking.
"We have an incredible decision to make, not just about the direction of America but the identity of our party and the conservative movement. The time for games is over," Rubio said in his closing statement.
"I know you had a lot of choices to make, but now it's time to narrow it down and I'm asking you to get behind me ... so we can bring an end to this silliness, this looniness."
Trump's counterattacks draw blood
An underestimated Trump quality: His counterpunches often play extremely well with conservatives who distrust politicians and the media.
For instance, an early exchange, when Rubio asserted that Trump is "the only person on this stage that's ever been fined for hiring people that worked on your projects illegally."
Trump's response: "I'm the only one on this stage that's hired people."
Another of his one-liners may have been the most damaging. "This guy's a choke artist, and this guy's a liar," he said, turning to first Rubio and then Cruz.
Expect to hear more of that in the days ahead. Time and time again, Trump has proven that he owns the post-debate.
Whether it's leveraging his massive influence on social media to wage a war against Fox's Megyn Kelly or driving home his best lines of attack by calling in to every news television show on the air, he has a way of shortening the half-life of bad headlines.
As soon as the debate ended, he mocked Rubio's perspiration.
"It looked like he just came out of a swimming pool. He was soaking wet," Trump told CNN's Chris Cuomo. "He's a meltdown guy. I mean I look at him, he's just pouring sweat. ... We need somebody that doesn't sweat."
Trump shows why he's winning
He was hit from both sides of the stage Thursday night, but Trump managed to score some points of his own.
He consistently owns some issues that none of his rivals fight him for.
Trump laid into Mexico and China, blasting U.S. trade policies and giving Americans a direct outlet for their anger about job losses and wage stagnation.
He used former Mexican President Vicente Fox's attack
in a Fusion interview, when he said he's "not going to pay for that f---ing wall" that Trump wants to build, to showcase his strength in the face of adversity.
"The wall just got 10 feet higher," Trump said.
Did Cruz do enough?
He spent much of his time attacking Trump, too -- but Cruz was clearly Robin to Rubio's Batman in going after the front-runner on stage.
The Texas senator's line of attack was designed to undercut Trump's conservative credentials. And if that was the goal, he had some success -- with Trump asserting at one point that "millions of women are helped by Planned Parenthood," a group that is anathema on the right.
Mostly, though, Cruz waited for openings that were never there -- because Rubio had spotted them first.
Cruz did regain his footing late in the debate, laying into Trump for donating to Democratic politicians and deflecting Trump's goading that he not "get nervous" by saying, "I promise, Donald, there is nothing about you that makes anyone nervous."
But the most raucous debate yet was about personality, and Cruz showed less of it than Rubio and Trump.
His best line might have come at the start of the debate.
"In 2013 when I was leading the fight against the 'Gang of Eight' amnesty bill, where was Donald?" Cruz said. "He was firing Dennis Rodman on 'Celebrity Apprentice.'"
Cruz's problem -- Rubio's attacks showed a new side of the Florida senator, and that may get him more of a second look than Cruz gets.
'Can someone attack me, please?'
That was Ben Carson's unsuccessful effort to work his way into an explosive exchange between Rubio, Trump and Cruz.
There were five candidates on stage. But Carson and Ohio Gov. John Kasich were both off to the side — their refusal to engage with other candidates, or criticize anything at all, turned both into afterthoughts.
Asked to judge his whether his opponents understand the importance of winning the support of Latinos, Kasich delivered a line that underscored his entire night, starting his answer by saying: "I'm not going to talk about that."
But Carson may own social media for another line. When it comes to choosing a Supreme Court nominee, Carson said, he would look examine "the fruit salad of their life