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 American public 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 fundraiser he held for Romney that "ruined" his carpet. Trump said Romney did not compensate him for the damages.

"Hey maybe I can send Mitt a bill for 'carpet ruined.'" Trump quipped.

Trump also called out Romney for turning on him after their 2012 endorsement event.

"He came out — it was very nasty. I thought he was a better person than that," Trump said of Romney's speech on Thursday. "So you help somebody, and then he turns. Now, I will say this. He probably had a right to turn, because nobody could've been nastier in getting him not to run [a third time for president] than me."

He added that Romney "doesn't have what it takes to be president."

Mitt Romney. AP

Trump and Romney have repeatedly traded barbs in recent days.

Romney had denounced Trump for dodging questions about the Ku Klux Klan and for not releasing his tax returns before his Thursday speech.

"A disqualifying & disgusting response by @realDonaldTrump to the KKK. His coddling of repugnant bigotry is not in the character of America," Romney tweeted on Monday after Trump said in an interview that he didn't know enough about former KKK Grand Wizard David Duke or white supremacists to denounce them.

Trump later attributed his answer to a faulty earpiece.

On taxes, Romney has repeatedly suggested Trump could be hiding a "bombshell" in his returns. He insisted Thursday that Trump must release his tax returns, and added that he thinks Trump doesn't do "much if anything" for disabled Americans and veterans.

"Well, I think there's something there," Romney told Fox News host Neil Cavuto last week. "Either he's not anywhere near as wealthy as he says he is, or he hasn't been paying the kind of taxes we would expect him to pay."

Trump, unleashed tirades against Romney on social media and in public in the days before his Maine rally.

Last week Trump tweeted that Romney was one of the "dumbest and worst" Republican candidates to ever run for president.

Watch Trump's comments from his Thursday rally in Maine below: