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No, we're not talking about Ted Cruz. The real estate success program is the center of four suits against Trump that claim he didn't deliver on his promises to students. "I guarantee you", Trump responded, an allusion to his manhood. It is going for $25 on tedcruz.org.

Polls show Trump leading in MI, though Cruz and Rubio are both campaigning hard. "I guarantee you there is no problem".

The debate began with the anchors bringing up Romney's sharp condemnation of Trump earlier Thursday calling him "a phony, a fraud" in a speech in Salt Lake City. The candidate embodies the "very brand of anger that has led other nations into the abyss", Romney said.

"Obviously, I'm going to disavow that", he said.

Instead it was Cruz who has argued this is now a two-man race, pointing to his wins in Texas, Oklahoma, Iowa and later Alaska over Trump.

No other candidate came close: Florida Sen.

Trump is "trying to do to the American voter what he did to the people that signed up for this course", Rubio said.

In the hours before the debate, Trump traded barbs with the Republican Party's 2012 nominee, former Massachusetts Gov. Mitt Romney. Those candidates are Marco Rubio, Ted Cruz, John Kasich and frontrunner Donald Trump.

While Cruz and Rubio never missed a chance to attack Trump, Kasich never missed a chance not to.

Also publicly criticizing Trump: House Speaker Paul Ryan, who was Romney's running mate, and 2008 GOP nominee John McCain.

"This little guy has lied so much", Trump sneered, mocking Rubio's stature. And Trump said he'd support one of them if he didn't get the nod - even though, he added, they don't deserve it.

Trump opened the debate with his devastating response to Rubio joking about the size of Trump's hands and saying "you know what they say about men with small hands".

The stakes are particularly high heading into March 15, when Rubio's home state of Florida will be among a handful of states to hold GOP contests.

"Let's see what happens at the end of three years", Trump said.

Some party leaders and donors are critical of Mr Trump's positions on trade and immigration, including his calls to build a wall between the USA and Mexico, deport 11 million illegal immigrants and temporarily bar Muslims from entering the country.