(CN) - Mitt Romney, the 2012 Republican president nominee, tore into party frontrunner Donald Trump Thursday in Utah, warning that a Trump presidency would cast the nation into recession, make the world less safe, and "will mean America will cease to be a shining beacon on a hill."
Romney's much-anticipated remarks, delivered before an auditorium packed with students at the University of Utah's Hinckley Institute of Politics, rested on assertions that Trump is "a phony" and "a fraud" who is "playing the American public for suckers."
In an extraordinary, 20-minute rebuke, Romney called on his fellow Republicans to reject the current Republican frontrunner's candidacy, warning that if they don't, his campaign "will have profound consequences for the Republican Party and the country."
Romney has been increasingly vocal about his distaste for Trump as the billionaire real estate developer and reality television star has racked up primary and caucus wins. He previously called on the candidate to release his income tax returns.
Two of Trump's opponents, Sen. Marco Rubio, of Florida, and Sen. Ted Cruz, of Texas, already have done so.
Trump has been slow to disclose his, explaining that they are "complicated."
But Romney continued to press Trump on the subject issue, predicting that the release of the tax returns and a transcript of an off-the-record interview Trump gave to The New York Times will be full of "bombshells."
Romney predicted the tax returns will reveal Trump "doesn't give anything to the veterans or the disabled," and that the transcript of the interview, which first came to light a few days ago, will reveal the candidate contradicting himself in private on his very public tough stand on illegal immigration.
If the public gets to see either the returns or a transcript of the interview, Romney said, "you'll have all the proof you need that Trump is a phony."
He added, "It is entirely in his hands to prove me wrong."
Romney then went on to assail Trump's campaign promises "as worthless as a degree from Trump University, " a broadside that comes a day after an appeals court in New York gave state Attorney General Eric Schneiderman more time to argue fraud claims against Donald Trump's Trump University.
"Trump likes to tell us he's a big success in business and that he knows what he's talking about," Romney said.
"He isn't and he doesn't. He inherited his business. He didn't create it."
Romney then ticked off a lengthy list of businesses among them Trump Airline, Trump magazine and Trump vodka that have all gone belly up.
"Now, not all of his policies are bad," Romney said, striking a momentary conciliatory. "He calls for rescinding Obamacare and of bringing jobs home to the United States ... but his prescription for doing these things is flimsy at best."
Romney then went on to characterize Trump's foreign policy proposals as "recklessness in the extreme."
"Mr. Trump's bombast is causing concern among our allies and only serving to inflame our enemies," he said.
"Donald Trump tells us he is very, very smart. ... I'm afraid when it comes to foreign policy, he is very, very not smart. ... dishonesty is Donald Trump's hallmark," Romney said, adding that the candidate's "greed," "misogyny," and "absurd, third-grade theatrics" are "no example for America's children and grandchildren."
Landing another insult and hecklers sporadically tried to disagree, Romney said, "Trump relishes any poll that reflects what he thinks of himself."
"But the polls also show that if he is the Republican candidate, he will lose ... to Hillary Clinton," Romney said.
And if he is the Republican candidate in the fall, Romney predicted, Trump's "infamous" exchange with CNN's Jack Tapper, in which the candidate failed to disavow the Ku Klux Klan, "will play a hundred times a day."
Despite his obvious distaste for Trump. Romney will stop short of endorsing any of the other Republican candidates in the race in remarks delivered at the university's Libby Gardner Hall. Instead, he'll take a more even-handed approach.
"Of the remaining candidates, the only serious policy proposals that deal with the broad range of national challenges we confront have come from Ted Cruz, Marco Rubio and [Ohio Gov.] John Kasich," he said.
Trump responded characteristically.
"I backed him [in 2012]. He was begging for my endorsement. I could have said, 'Mitt, drop to your knees,' and he would have dropped to his knees," he said while campaigning in Portland, Maine Thursday afternoon.
"He's a choke artist," Trump added. "He should have won four years ago, and instead he disappeared."
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