GOP presidential candidates debate following Trump-Romney fight

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DETROIT -- Republican presidential front-runner Donald Trump is taking the debate stage Thursday night after spending the day in a heated war of words with the party's 2012 nominee, Mitt Romney.

Romney, who has blasted Trump for days, delivered a sharp condemnation of Trump earlier Thursday.

"Here's what I know: Donald Trump is a phony, a fraud," Romney said in a speech in Salt Lake City. "His promises are as worthless as a degree from Trump University. He's playing members of the American public for suckers. He gets a free ride to the White House, and all we get is a lousy hat."

Trump hit back during a speech later in the day in Maine, blasting Romney for "begging" for his endorsement.

"I don't know what happened to him," Trump said. "You can see how loyal he is. He was begging for my endorsement. I could have said, 'Mitt, drop to your knees.' He would have dropped to his knees."

The debate comes as the GOP field is quickly winnowing. When Trump took the debate stage for the first time in August, 17 candidates were seeking the Republican nomination. Eight months later, Trump is the undisputed front-runner and will be on stage with just three rivals: Marco Rubio, Ted Cruz and John Kasich.

The prime-time event comes two days after Trump dominated the Super Tuesday contests, notching seven victories and picking up hundreds of new delegates.

Cruz, who kicked off the GOP primary season by winning the Iowa caucuses, added three victories on Tuesday, including in his delegate-heavy home state of Texas. Rubio also got his first win of the season, coming in first in Minnesota's caucuses.

With Trump emerging as the undisputed GOP front-runner, many party leaders and insiders insist he's not a true conservative. They also worry that Trump's inflammatory rhetoric on immigration could permanently unravel the GOP's efforts over the years to reach out to Latinos and other minorities.

But even as Romney and others try to lead the effort to stop Trump, the belated push may only reinforce the real estate mogul's appeal as his supporters have already shown their deep loathing for establishment figures.

For Cruz and Rubio, the window to catch up with Trump in the delegate count is quickly closing. 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.

Both senators are jockeying to be the last man standing in the GOP field to take on Trump and are expected to take on not only Trump -- but each other -- on Thursday.

Meanwhile, Kasich is coming under pressure to ditch his presidential bid.

The Ohio governor, who came in second in New Hampshire by running on a moderate message, hasn't had a single victory in the 2016 election, and some Republicans are accusing Kasich of taking away valuable support from Rubio.