Rubio in particular came out hot, seemingly dumping his opposition research on Trump as he attacked the billionaire on his eponymous university, his business record and his core political beliefs.
Trump, for his part, mostly kept his cool through the two-hour debate in Houston even as he was the focus of relentless attacks from Cruz and Rubio.
Both Cuban-American senators are trailing Trump badly in the delegate race, and need to do something fast to turn around the race.
A dozen states and U.S. territories will hold contests on Tuesday in the biggest day of the primary yet. Trump is leading polls in a majority of states, and has dominated the last three contests, in New Hampshire, South Carolina and Nevada.
As a result, Cruz and Rubio were desperate on Thursday to take Trump down a peg. The front-runner, for his part, dished out a series of counter-attacks while repeatedly returning to a singular theme: that he was the one non-politician on the stage who could provide answers for the nation.
He also largely dialed back his stage presence, seemingly content to run out the clock and protect his lead.
Rubio had the best attack line of the night while badgering Trump about his plan is to replace ObamaCare.
Trump responded by repeatedly saying the centerpiece of his plan would be to allow insurers to cross state lines.
“Now he's the one repeating himself,” the Florida senator said, cracking a big smile as the crowd erupted in cheers and laughter.
Like most bullies, Trump showed that he can dish out the insults, but that he can't take them at all (which is why I'm not too worried about him taking on Hillary or Bernie, they'll smash his mouth a few times and he'll fall apart like he did last night.) But it was an abysmal affair even by GOP debate standards, with Wolf Blitzer and Hugh Hewitt losing any semblance of control roughly three seconds into the proceedings.
But the line of the night? Sleepy Dr. Godwin!
“Can somebody attack me, please,” Carson pleaded at one point.
These guys don't have a chance in hell.