NEW YORK - US Senators Marco Rubio and Ted Cruz unleashed their harshest attacks yet at Donald Trump on Thursday night in Houston, Texas, trying to slow the billionaire businessman's momentum before Super Tuesday's primaries, which could vault him far ahead in the race for the Republican presidential nomination.

The CNN-hosted debate at the University of Houston was the two first-term senators' last, best chance to try to shake up the race for the Republican nomination. The contest is dramatically shifting toward Trump, who is leading in opinion polls in nearly all 11 states set to make their choices on next Tuesday.

But Thursday's 10th debate at times deteriorated into personal, vindictive bickering. The national election are scheduled for Tuesday, November 8.

“Donald, relax,” Cruz, the ultra conservative, said during one dispute Thursday night.“I'm relaxed,” Trump responded. “You’re the basket case.”

The two US senators ganged up on Trump on a slate of topics — immigration, health care, his family money, his personal finances, and basic fitness for the presidency — during the CNN event in Houston. It was a change of pace for Rubio, who has generally avoided direct conflict with Trump, and it made this the most contentious debate yet.

Cruz questioned whether Trump is even a Republican, citing his past support for Democratic policy positions and candidates, including Hillary Clinton.

Rubio also ridiculed Trump for claiming he's the one that introduced the topic of immigration reform to the 2016 race, blasting him as "only person on this stage that has ever been fined for hiring people to work on your projects illegally."

Trump fired back that he was the only one on stage who had hired anyone, period. The two other Republicans left in the race, former Ohio Governor John Kasich and retired neurosurgeon Ben Carson, were left fighting to get a word in. Carson at one point begged for speaking time, noting the debate rules that candidates could respond to attacks.

“Can somebody attack me, please?” Kasich prodded the moderators to take control of the debate and end the bickering.

Rubio referred to Trump's now-defunct Trump University as a “fake” school: “You know what they got? They got to take a picture with a cardboard cutout of Donald Trump.” Cruz picked up on this attack and predicted a pending fraud trial brought by students would derail Trump's campaign later this year.

On Trump’s suggestion to put a high tariff on Chinese exports to the United States, Rubio mocked the businessman: “You are going to be starting a trade war against your own ties and your own suits.”

The attacks were an effort to dent the commanding position Trump is in after his easy wins in New Hampshire, South Carolina and Nevada, and strong second-place finish in Iowa. The latest delegate count: Trump 82, Cruz 17, Rubio 16, John Kasich 6 and Ben Carson 4. Next week on Super Tuesday, 595 delegates are up for grabs — about half of the 1,237 necessary to win the Republican nomination.

CNN moderator Wolf Blitzer asked Trump about his tax returns, noting that 2012 Republican nominee Mitt Romney had raised the issue Wednesday and taunted him on Twitter on Thursday to “show voters your back taxes.” Trump said he routinely gets audited by the IRS. “Every year they audit me, audit me, audit me,” Trump said. He said he wants to share his tax history, but “I can’t do it until the audit is finished, obviously.”Trump’s case against Rubio and Cruz was that they’re both inept politicians, who either melt down under pressure or mislead voters.“You are all talk and no action. This guy’s a choke artist,” he said, referring to Rubio, “and this guy’s a liar” referring to Cruz.

Trump added: “Other than that, I rest my case.”

Some Republican leaders are anxious about the prospect of a Trump nomination, arguing that his comments about women and Mexican immigrants, as well as his attacks on Republican opponents, will hurt the Republican ticket in the fall. Criticisms have included Trump’s support of torture and his recent comment at a rally that he would like to punch a protester in the face.

But Trump sees a chance to defeat three of his rivals in their home states: Cruz in Texas on Tuesday, Rubio in Florida on March 15, and Kasich in Ohio, also on March 15. Any of those would likely be a fatal blow.