Republican presidential candidate Donald Trump responded to Mitt Romney's searing broadside on Thursday with one of his favorite put downs in the race for the GOP nomination.

"He's a choke artist," Trump said at a rally in Portland, Maine, referring to the former GOP presidential nominee's failure to defeat President Barack Obama in 2012. "We cannot take another loss. Mitt is indeed a choke artist."

Romney lambasted Trump earlier in the day, calling the businessman a "phony" and a "fraud," and accused him of "playing the American public for suckers." The speech was a stunning development and the clearest sign yet that the GOP is fracturing over its new front-runner after his dominating performance on Super Tuesday.

In his speech on Thursday, Trump called Romney "a failed candidate" who "failed horribly" and claimed he "begged" for his endorsement during the 2012 presidential race.

"I could have said, 'Mitt drop to your knees.' He would have dropped to his knees," Trump said, to laughter from the audience.