Fox News host Megyn Kelly may as well have been on the debate stage at Thursday's Republican presidential debate in Detroit, because she wiped the floor with real estate mogul Donald Trump over a 2004 business venture that has recently come under scrutiny.

The unaccredited Trump University took an estimated $40 million from some 7,000 students, and some are now suing Trump for fraud in two California class-action lawsuits, along with a separate case being pursued by New York's attorney general.

Trump's opponents have used the controversy to attack him on the campaign trail. Former GOP presidential nominee Mitt Romney on Thursday cited it to accuse Trump of being a "fraud" and a "con artist" who is "playing the American people for suckers."

Kelly put Trump in the hot seat over the venture, pointedly grilling him over facts of the case and flashing on the television screen key claims from plaintiffs alleging wrongdoing. 

"They found victims of con artists sing the praises of their victimizers until they realize they have been fleeced," Kelly said.

Trump, however, brushed aside the questioning.

"Let's see what happens in court," Trump responded. "This is a civil case. Very easy to settle. Could settle it now. Very easy to have settled. Let's see what happens at the end of a couple years when this case is over, okay?"

Sen. Marco Rubio (R-Florida) jumped in, accusing Trump of running a scam.

"They signed up. Paid $15,000. You asked for additional money for the course. If they wanted the real secrets of success, they had to pay more money, so they did. You know what they got in the courses? Stuff you can pull off Zillow," Rubio said, referring to the real estate website. "When they realized what a scam it was, they asked for their money back and refused to give their money back."

"I gave many people their money back," Trump responded.

He added: "We will see who's right at the end of a few years. Almost all of the people, many, many people signed what's called the report at the end, did you like the course, how did you like it? Almost all of them said it was terrific, okay?"