David Jackson

USA TODAY

Mitt Romney jumped into the Republican civil war over Donald Trump on Thursday, blasting the front-runner for the GOP presidential nomination as a "phony" and a "fraud" who would hand the White House over to Hillary Clinton in the fall election.

"His promises are as worthless as a degree from Trump University," the 2012 Republican nominee said in a heavily promoted speech at the University of Utah. "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."

Romney, who did not endorse any of Trump's campaign rivals, echoed a 1964 speech by Ronald Reagan by saying that Republicans "face another time for choosing" when it comes to the prospect of a Trump nomination.

Trump, who endorsed Romney in 2012 and is now on the cusp of the Republican nomination himself, fired back at a rally in Portland, Maine, where his supporters booed Romney's name.

"Mitt was a disaster as a candidate," Trump said.

Trump also noted he endorsed Romney in 2012 — "you can see how loyal he is" — and that the former Massachusetts begged for his support and "would have dropped to his knees" to get it.

In his blistering speech, delivered just hours before a Republican debate in Detroit, Romney — a Michigan native whose father was governor of the state — said that Trump's domestic plans would plunge the nation into recession and his foreign policy would alienate allies and undermine the fight against terrorism.

The former Massachusetts governor hammered Trump for his attacks on other Republicans, his use of expletives, his ego, his bragging about extramarital affairs, his recent reticence to criticize the Ku Klux Klan, and his "third-grade theatrics."

Romney did not mention that he accepted Trump's endorsement for his own presidential campaign in 2012, and praised the New Yorker's business acumen at that time.

Romney's speech, which echoed criticism of Trump by rivals Ted Cruz, Marco Rubio and John Kasich, comes in the wake of a series of Trump wins on the campaign trail, many of them fueled by Tea Party backers and new voters who are critical of the Republican establishment.

As other candidates step up their attacks on Trump, a number of Republican-leaning organizations are developing their own campaigns designed to stop Trump's nomination, either before or during the July convention in Cleveland. Romney suggested Republicans should deny Trump delegates by voting in each state for the candidate with the best chance to beat him there, including Rubio in Florida and Kasich in Ohio.

GOP leaders seek to halt Donald Trump's momentum

Another former Republican presidential nominee — John McCain, who lost to President Obama in 2008 — said he shared many of Romney's concerns about Trump, citing the businessman's "uninformed and indeed dangerous statements on national security issues."

Romney also had harsh words for Clinton, this year's Democratic front-runner, saying that "America's interests were diminished in every corner of the world" during her tenure as secretary of State. While Clinton "jettisoned her most profound beliefs to gain presidential power," Romney said that a Republican nomination of Trump "enables her victory" in the fall, citing recent polls.

Romney noted that Trump has praised Russian President Vladimir Putin and attacked Republican President George W. Bush, calling that a "twisted example of evil trumping good." He also referenced Trump's harsh comments about women, Muslims, and Mexican migrants, as well as what he called unrealistic proposals to have Mexico build a wall along the southern U.S. border.

"His domestic policies would lead to recession," Romney said. "His foreign policies would make America and the world less safe. He has neither the temperament nor the judgment to be president. And his personal qualities would mean that America would cease to be a shining city on a hill."

Trump kept up his rebuttal of Romney all day on social media.

"I am the only one who can beat Hillary Clinton," Trump tweeted. "I am not a Mitt Romney, who doesn't know how to win. Hillary wants no part of 'Trump.'"

On NBC's Today show, Trump said, "Mitt Romney is a stiff."

Romney has been attacking Trump for several days over his failure to release his taxes and other issues. He has posted only anti-Trump messages on Twitter for the past week. In his speech, Romney said, "I predict that there are more bombshells in his tax returns."

Trump's son Eric tweeted that Romney was showing no loyalty to Trump for his 2012 endorsement.