Marco Rubio and Ted Cruz goes on the attack against Donald Trump in GOP debate

HOUSTON, Tx. -- Brawling from the get-go, a fiery Marco Rubio went hard after Donald Trump in Thursday night's Republican debate, lacerating the front-runner's position on immigration, his privileged background, his speaking style and more.

Ted Cruz piled on, too, questioning the front-runner's conservative credentials, as the two senators tag-teamed Trump in a debate that reflected the increasing urgency of their effort to take down the billionaire businessman before he becomes unstoppable. It was a rare night where the bombastic Trump found himself on the defensive.

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 Super Tuesday 11-state round of mega-voting that could all but lock up the nomination.

When Trump faulted Rubio on a deal to buy a $179,000 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 criticized 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 $1 million fine for illegal immigration hiring.

The candidates were pressed on why they haven't released their tax returns as promised. The GOP's 2012 nominee, Mitt Romney, suggested this week that Trump was holding back because there was a "bombshell" that would be revealed.

Thursday night, Trump offered a new excuse, saying that he's been audited by the IRS every year and can't release his returns while that's going on.

Rubio and Cruz both promised to release more of theirs in the next two days.

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 criticized 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 apologize," 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."

When Trump repeated his positions on health care, Rubio pounced again.

"Now he's repeating himself," Rubio said with a grin.

Trump was hardly silent, responding to both Rubio and Cruz: "This guys a choke artist and this guy's a liar. ... Other than that I rest my case."

While Rubio was loaded for bear from the start, 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.

Making light of Cruz's attempts to diminish him, Trump said: "Keep fighting, keep swinging, man, swing for the fences."

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 GOP debate of the campaign, clearly showing the growing sense that Trump is on track for the nomination.

The debate's location in Houston gave a nod to the primacy of Texas in the Super Tuesday voting: There are 595 delegates at stake, 155 in Texas.

The debate audience included former President George H.W. Bush, 91, and his wife, Barbara - who missed out on the chance to see their son Jeb take part. He dropped out of the race after a poor showing in the first states to vote.

The other two remaining candidates, Ben Carson and John Kasich, were largely left to watch the fireworks flying overhead.

At one point, as the top three candidates mixed it up, Carson spoke up: "Can somebody attack me please."

Later, he complained, "I didn't get asked about taxes, I didn't get asked about Israel." When all five were asked about North Korea's president, he said, "We should make sure that he knows that if he ever shoots a missile at us it'll be the last thing he does."

Kasich, for his part, said he would try to find a way to effect regime change in North Korean but "perhaps the Chinese can actually accomplish that."