Mitt Romney. AP Photo/Manuel Balce Ceneta Mitt Romney railed on GOP presidential frontrunner Donald Trump in a major address Thursday, calling him a "phony," "fraud," "con man," and "fake."

"Let me put it plainly," he said. "If we Republicans choose Donald Trump as our nominee, the prospects for a safe and prosperous future are greatly diminished."

Romney's speech came as the Republican establishment grows increasingly worried about the growing possibility of a Trump nomination. It was delivered two days after Trump scored a dominating victory in the Super Tuesday series of primaries and caucuses.

Romney's speech — remarkable in that the party's most recent nominee was launching an unprecedented assault against its current frontrunner — went after Trump's business history, policy proposals, and fitness for office.

Listing many of Trump's previous businesses, like Trump Steaks, Trump Airlines, and the controversial Trump University, Romney said Trump wasn't a "business genius."

After then speaking about a remark Trump made on "60 Minutes," in which Trump said the US should let ISIS overthrow Syrian President Bashar Assad, Romney called Trump's foreign policy "reckless."

"Donald Trump tells us that he is very, very smart," Romney said. "I'm afraid that when it comes to foreign policy he is very, very not smart."

"Here's what I know: Donald Trump is a phony, a fraud," Romney said.

"His promises are as worthless as a degree from Trump University," he continued. "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."

Mitt Romney. Screenshot/YouTube Romney, who recently has been publicly critical of Trump's campaign, gave his Thursday-morning speech at the Hinckley Institute of Politics at the University of Utah in Salt Lake City.

"His domestic policies would lead to recession," Romney said later. "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."

He went on to say that if Trump were to win his party's nomination, the election would be handed to a "dishonest" Hillary Clinton, the former secretary of state who is the Democratic frontrunner.

"Trump relishes any poll that reflects what he thinks of himself," he said. "But polls are also saying that he will lose to Hillary Clinton."

Romney added that Trump's Republican challengers Gov. John Kasich of Ohio and Sens. Ted Cruz of Texas and Marco Rubio of Florida were the only candidates with "serious policy proposals."

The 2012 GOP presidential nominee also channeled former US President Ronald Reagan in his speech, mentioning a 1964 address Reagan made about the future of the country.

"I'm no Ronald Reagan and this is a different moment, but I believe with my heart and soul that we face another time for choosing, one that will have profound consequences for the Republican Party and more importantly, for the country," he said.

Romney also called out Trump for calling former US President George W. Bush a "liar," and he said Trump "admires" Russian President Vladimir Putin.

"That is a twisted example of evil trumping good," he said.

Mitt Romney. Screenshot/YouTube He went on to say he understood the anger felt by the national electorate — anger that has led to Trump's rise within the party. But, Romney — in another slight at Trump — said that, in the past, anger from voters was "transformed into energy directed for good" by past presidents.

Trump, he insinuated, wouldn't be the best example for future generations.

"The president and yes, the nominees of the country's great parties help define America to billions of people," Romney said. "All of them bear responsibility of being an example for our children and grandchildren."

Romney in recent days has been locked in a back-and-forth with Trump, whom he's denounced for dodging questions about the Ku Klux Klan and for not releasing his tax returns.

"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 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. Or perhaps he hasn't been giving money to the vets or the disabled, like he's been telling us he's been doing."

Mitt Romney. Screenshot/YouTube Trump, for his part, has unleashed multiple tirades against Romney on social media and in public.

"Watch how he responds to my speech today," Romney said Thursday. "Will he talk about our policy differences, or will he attack me with every imaginable low-road insult? This may tell you what you need to know about his temperament, his stability, and his suitability to be president."

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

"I criticized Mitt Romney for losing that election. He should have won that election," Trump said during last week's GOP debate in Houston.

After bits of Romney's speech were leaked early Thursday morning, Trump took to Twitter to unleash a scathing series of tweets at the former governor.

With primary season sailing along, the real-estate mogul leads Cruz, who is in second place, by nearly 100 delegates, and he leads Rubio by more than 200 delegates. Trump is also leading the polls in Michigan, Ohio, Florida, North Carolina, and Illinois, the five largest states that hold elections in the next two weeks. Trump captured at least seven of the states that held primaries or caucuses on Super Tuesday.