Republican U.S. presidential candidates Marco Rubio and Donald Trump both speak at the debate sponsored by CNN for the 2016 Republican U.S. presidential candidates in Houston, Texas, Feb. 25, 2016. REUTERS/Mike Stone

Florida Sen. Marco Rubio came out strong against GOP front-runner Donald Trump early in Thursday's Republican debate in Houston, taking jabs at the billionaire for his stances on immigration and Israel. Rubio took aim at Trump for hiring workers from outside the U.S., alleging that the candidate had been fined for “hiring people to work under projects illegally.”

Trump fired back. “I’m the only one on stage that’s hired people,” he said. “You haven’t hired people. I’ve hired tens of thousands of people. You’ve done nothing. … I’ve hired tens of thousands of people.”

He also called out Trump for repeating himself during the debate, an attack previously leveled at Rubio himself. Trump said he had seen Rubio "repeat himself five times four weeks ago." Responding, Rubio said: "I saw you repeat yourself five times five minutes ago."

After the first half of the debate, however, social media users noted that Rubio had gone quiet. But asked about Trump's stances on Israel, Rubio came back with attacks against Trump for his purported balanced stance in relation to the longstanding conflict with the Palestinians. "This is not a real-estate deal," Rubio said, arguing that the U.S. must stand strongly with Israel.

Rubio has struggled to pull ahead of GOP front-runner Donald Trump and Texas Sen. Ted Cruz, but was given a boost recently by the South Carolina primary, after he came in second, narrowly beating Cruz. The win pressured former Florida Gov. Jeb Bush — once the expected nominee — out of the 2016 presidential race.

"This has become a three-person race, and we will win the nomination," Rubio told his supporters the evening results were released.

The debate Thursday also comes after Rubio took second in Nevada, with 24 percent, narrowly beating Cruz, who held 21 percent of the vote. Trump, who won a sharp victory, is heading confidently into Super Tuesday next week, when voters in a series of states, including Texas, Alabama, Arkansas, Georgia and Tennessee, will head to polls and caucuses.

Rubio is hoping that bitter dislike among the GOP establishment for Trump will translate into votes for him. His campaign has been challenged by questions of his readiness for the position, particularly his limited experience in foreign policy. Thursday's CNN-Telemundo debate might be his chance to show primary voters that he's a viable candidate.