Brawling from the get-go, a fiery Marco Rubio went hard after Donald Trump in a Republican presidential debate, attacking the front runner's position on immigration, his privileged background, his speaking style and more.
Ted Cruz piled on too, questioning the billionaire's conservative credentials, as the two senators tag-teamed Trump in the debate.
It was a rare night where the bombastic Trump found himself on the defensive.
The other two remaining candidates, Ben Carson and John Kasich, were largely left to watch the fireworks flying overhead.
The debate played out as a raucous night of tit-for-tat insults, with candidates shouting over one another so much that it was hard to follow at times.
The showdown came just days before the March 1 Super Tuesday 11-state round of mega-voting that could all but lock up the nomination.
Trump may well become the inevitable Republican candidate after Super Tuesday, where 595 delegates are at stake.
So far, after four primary and caucus contests, Trump has 82 delegates, Cruz has 17 and Rubio has 16. A candidate must have 1237 state delegates to win the Republican nomination at the party's convention this summer.
When Trump faulted Rubio on a deal to buy a $US179,000 ($A247,545) house, the Florida senator shot back that if Trump "hadn't inherited $200 million, you know where Donald Trump would be right now? Selling watches in Manhattan."
In another rough exchange, Rubio accused Trump of shifting his position on deportation, hiring people from other countries to take jobs from Americans and being fined for worker violations. Joining in, Cruz criticised Trump for suggesting he alone had "discovered the issue of illegal immigration".
Trump shot back at Rubio: "I hired tens of thousands of people. You've hired nobody."
As for Cruz, Trump took a more personal tack, touting his own ability to get along with others and adding: "You get along with nobody. ... You should be ashamed of yourself."
Both Rubio and Cruz said that Trump had had to pay a $US1 million fine for illegal immigration hiring.
Rubio was the principal aggressor of the night, and he held nothing back.
Taking on Trump's declaration that he'd build a wall on the Mexican border, Rubio declared: "If he builds a wall the way he built Trump Tower he'll be using illegal immigration to do it."
Trump, for his part, insisted that even though officials in Mexico have said they won't pay for his planned wall, "Mexico will pay for the wall". And he said that because Mexico's current and former presidents had criticised him on the issue, "the wall just got 10 feet taller".
Trump, known for his frequent use of coarse and profane language on the campaign trail, scolded former Mexican president Vicente Fox for using a profanity in talking about Trump's plan for the wall.
"He should be ashamed of himself and he should apologise," declared Trump.
After Trump mocked Rubio for his "meltdown" in a previous debate when the Florida senator repeated rote talking points, Rubio swatted right back, scolding Trump for spouting the same things over and over Thursday night: "Everyone's dumb. He's going to make America great again. We're going to win, win, win. He's winning in the polls."
Trump was hardly silent, responding to both Rubio and Cruz: "This guy's a choke artist and this guy's a liar. ... Other than that I rest my case."
Cruz ramped up his criticism as the night word on. The Texas senator said Trump wasn't releasing his tax returns "because he's afraid" and he argued that Trump wouldn't be an effective opponent against Hillary Clinton in the general election.
In the past, Rubio and Cruz had shown little willingness to take on the former reality television star when the national spotlight was the brightest. That all changed in the ninth Republican debate of the campaign, clearly showing the growing sense that Trump is on track for the nomination.