The Republican presidential candidates are sparring in the last debate before Super Tuesday when 11 states will vote.
Senators Marco Rubio and Ted Cruz stepped up their attacks frontrunner Donald Trump on immigration issues.
The New York billionaire has won three out the first four contests, including most recent vote in Nevada.
Mr Rubio rolled out a new line of attack, saying that Mr Trump in his businesses employed foreign workers rather than Americans.
Mr Trump shot back: "I hired tens of thousands of people. You've hired nobody."
Mr Trump has been extremely popular despite his controversial comments about banning Muslims from travelling to the United States and building a wall along the Mexican border.
After four primary and caucus competitions, Mr Trump has 82 Republican party delegates, Senator Ted Cruz has 17 and Marco Rubio has 16. To become the Republican party's nominee, a candidate has to have 1,237 total state delegates.
Former neurosurgeon Ben Carson has said he will not be quitting the race anytime soon, despite a poor showing in polls and in state contests so far.
Ohio Governor John Kasich, billing himself as a moderate, compassionate alternative to his more conservative colleagues, has been pressured to drop out by some Republicans so a non-Donald Trump frontrunner can emerge.
Mr Cruz placed third in three consecutive state races and Mr Rubio had a major stumble in a debate before the New Hampshire primary. He repeated himself several times and was ridiculed online.
Mr Trump has said he is confident heading into Super Tuesday.
"It's going to be an amazing two months," he said after winning the Nevada contest on Tuesday.
"We might not even need the two months, folks, to be honest."
He has gone after Mr Cruz recently, calling him a "liar" and questioning his campaign strategies.
Mr Rubio, meanwhile, is seeking the backing of establishment Republicans and has said the bid for the nomination is now a "three-person race".
