Donald Trump. AP Photo/Robert F. Bukaty GOP frontrunner Donald Trump responded to Mitt Romney's broadside on Thursday by saying that the former Republican presidential candidate had "begged" for his endorsement.

"He was begging for my endorsement," Trump said at a rally later in Portland, Maine.

"I could've said, 'Mitt, drop to your knees.' He was begging me," Trump said.

Trump was referring to the 2012 race for president in which Romney, the GOP nominee that year, sought Trump's endorsement.

But if they were friendly four years ago, that relationship has clearly deteriorated.

Earlier Thursday, Romney gave a speech in which he railed against Trump, whom he called a "fraud," "con man," "phony," and "fake," among other things.

"His promises are as worthless as a degree from Trump University," Romney jabbed. "He's playing the Americanpublic for suckers. He gets a free ride to the White House, and all we get is a lousy hat."

Throughout Trump's subsequent speech in Maine, the real-estate developer repeatedly and extensively lashed back out at Romney.

"Mitt is a failed candidate — he failed badly," Trump said. "That is a race that should have been won."

He repeatedly called Romney a "choke artist" throughout the speech, an insult he's used against Marco Rubio, a top Trump rival for the 2016 nomination.

"He's a choke artist, I started hitting him so hard," Trump said. "We can't take another loss. He choked, he choked like nobody I've ever seen except for Rubio," he continued.

At other points in his speech, Trump called Romney a "disaster" candidate in 2012. The billionaire businessman then mentioned a