PORTLAND, Maine — Hours after being attacked as a "fraud" by Mitt Romney, Republican presidential candidate Donald Trump slammed him as a "choke artist" and "failed candidate" who begged for Trump's endorsement during his 2012 presidential bid.

"You can see how loyal he is," Trump told a crowd of about 500 packed into a hotel ballroom here. "He was begging for my endorsement. I could’ve said, 'Mitt, drop to your knees,' and he would’ve dropped to his knees. He was begging. True. True. He was begging me."

Trump also derided Romney as "a failed candidate. He failed horribly. That was a race, I have to say folks, that should have been won.”

As Trump’s grip on the GOP nomination has tightened after winning seven primaries on Super Tuesday this week, the Republican establishment has become increasingly hostile to his candidacy. They fear that Trump’s hard-line stance on immigration reform and his anti-Muslim rhetoric could not only torpedo the party’s chances of beating the Democratic nominee in the fall but set back the party's brand for many elections to come.

Trump said Romney let the party down in 2012 during his race against President Obama, in which Trump endorsed Romney and recorded robo-calls on his behalf. The billionaire also spent several minutes telling supporters that he is wealthier than Romney.

“Mitt is indeed a choke artist,” Trump said.

Trump's remarks followed a scathing speech delivered in Salt Lake City earlier Thursday by Romney, who said the GOP front-runner has neither the temperament nor the qualifications for the Oval Office. Romney also disparaged Trump’s character, speaking in hostile terms about the man who once served as a supporter and high-profile surrogate during his own 2012 presidential bid.

"Donald Trump is a phony, a fraud," Romney said at the University of Utah on Thursday. "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."

Sen. John McCain (R-Ariz.), the party’s presidential nominee during the 2008 election, released a statement Thursday echoing Romney’s concerns.

“I want Republican voters to pay close attention to what our party's most respected and knowledgeable leaders and national security experts are saying about Mr. Trump, and to think long and hard about who they want to be our next Commander-in-Chief and leader of the free world,” McCain said in the statement.

Even as Republican leaders have been increasingly vocal in denouncing Trump, he has shown remarkable strength at the ballot box, building a coalition of voters across the country that appears as deep as it is wide. Trump not only secured a series of victories in Deep South states such as Georgia and Alabama on Tuesday, but he also won moderate Northeastern states such as Massachusetts and Vermont. Last month, he overwhelmingly won the Nevada caucuses in the West.

Sens. Ted Cruz of Texas and Marco Rubio of Florida have emerged as Trump’s primary rivals for the 2016 nomination and have torn into the billionaire in an attempt to prevent his march to the nomination. They have increasingly characterized Trump as a false conservative who is fooling the Republican electorate with empty rhetoric on immigration and foreign security that he is not qualified to act upon.

Trump dismissed Rubio on Thursday as having "the worst record in the history of Florida."

Maine Gov. Paul LePage — who endorsed Trump last week shortly after New Jersey Gov. Chris Christie announced his support — introduced the mogul onstage. Many in the crowd booed when the combative LePage took the stage but soon began cheering when he began complimenting Trump.

“The establishment has left us behind," LePage said. "The gridlock is so bad nothing gets done in Washington. And now the establishment is trying to defeat him."

Maine's Republican caucuses will be held Saturday.

Trump’s remaining Republican rivals — also including Ohio Gov. John Kasich, who has not won any primaries — will meet tonight for the 11th Republican presidential debate, in which Trump is expected to be the overwhelming target. Trump said Thursday that he will not hesitate to hit them back with his signature bombast.

“When you have incoming you can’t be too presidential,” Trump said. “They said, ‘Act presidential tonight.’ But if somebody hits me I’m going to hit them back harder.”