Marco Rubio clashed early and often with Donald Trump at Thursday’s GOP debate, trying to throw the primary front-ruer off his footing by bringing up a slew of controversies including his past alleged hiring of illegal immigrants and a “fake university” bearing his name.

The Florida senator cited the ongoing fraud case against the former Trump University, calling it a “fake school.”

The two candidates practically shouted over one another, until Trump asked the moderator if he was allowed to respond.

Rubio also invoked a case about Trump allegedly using illegal workers from Poland after initially criticizing Trump for hiring “a significant number of people from other countries to take jobs that Americans could have filled.”

“I‘m the only one on the stage that’s hired people. You haven’t hired anybody,” Trump shot back, calling Rubio’s allegation “wrong.”

But Rubio stood by it, and Cruz backed him up.

Rubio later said Trump “lied about the Polish workers.”

“Thirty-eight years ago,” Trump said.

“I guess there’s a statute of limitations on lies,” Rubio said.

According to press reports from more than 25 years ago on the case, Trump at the time claimed he didn’t knowingly hire undocumented workers.

At the debate, Trump also doubled down on his promise to build a U.S.-Mexico wall if elected, responding to Idea by former Mexican President Vicente Fox, who said Mexico will not pay for the “f------ wall.”

Trump said at the debate, “I will, and the wall just got 10 feet taller, believe me.”

Rubio again used the issue to bring up the illegal hiring case.

“If he builds the wall the way he built Trump Towers, he’ll be using illegal immigrant labor,” he said.

“Such a cute sound bite,” Trump responded.

The candidates faced off as Trump’s rivals scramble to undercut his momentum in a last-ditch bid before Super Tuesday to mount a viable challenge against the billionaire front-ruer.

The debate in Houston, hosted by CNN and Telemundo, is their last before more than a dozen states hold contests on Tuesday, when nearly half of the delegates needed to win the nomination are on the line. Anything close to a sweep by Trump would be devastating for the other remaining contenders.

The front-ruer’s momentum has only intensified this month with three straight primary and caucus wins, and he’s threatening to knock down his rivals, one by one, in each of their home states in March.

Ohio Gov. John Kasich’s campaign, despite performing poorly in the South Carolina and Nevada contests, issued a defiant call Thursday for Marco Rubio to drop out – citing his shaky numbers in his home state.

“@marcorubio needs to do the right thing & suspend campaign,” Kasich spokesman Chris Schrimpf tweeted, referring to the March 15 primary in Florida.

The state is considered a must win for the Florida senator, but a new Quiipiac University poll showed Trump leading Rubio 44-28 percent among primary voters there.

But Rubio, despite not having won a primary contest yet, has been racking up endorsements from fellow lawmakers and big-name Republicans, trying to position himself as the best candidate to win a general election in November and the best choice to take on Trump.

Rubio, who has finished second in each of the last two nominating contests, acknowledged during a Fox News forum with host Megyn Kelly on Wednesday that Trump is "the frontruer and I'm the underdog, but I've been an underdog my entire life."

At the same forum, Texas Sen. Ted Cruz argued that he – not anybody else – is the best alteative candidate since he’s the only other one who has won a primary contest, the Iowa caucuses.

He said his campaign is "the only campaign that can beat Donald [and] has beat Donald."

Trump, though, all along has argued that if other candidates drop out, he would pick up some of their support as well.

Ben Carson, meanwhile, has appeared immune to the political pressure or the poor showings in South Carolina and Nevada. After placing fourth in Nevada, Carson declared to supporters that “things are starting to happen.”