Mitt Romney launched a broadside against Donald Trump’s surging campaign on Thursday, calling the man who wishes to succeed him as the GOP’s presidential nominee a “phony” and a “fraud” who is playing Americans for “suckers.”

The 2016 front-runner fired back, calling Romney a "failed candidate" who begged him for his endorsement when he ran four years ago. Trump indeed endorsed Romney in the 2012 race.

Romney, though, delivered a scathing speech at the University of Utah about the prospect of a Trump presidency, in what amounted to perhaps the most full-throated effort yet by a member of the so-called Republican establishment to thwart Trump’s march to the nomination.

As one businessman to another, Romney even mocked Trump’s record of building companies, rattling off some of his lesser-known ventures like Trump Mortgage and saying he’d be bad for American workers.

“A business genius he is not,” he said.

More broadly, Romney called this a “time for choosing.” The party’s 2012 presidential nominee issued a stark warning that Trump’s policies would lead to recession and “make America and the world less safe” -- and said the “only serious policy proposals” are coming from the other Republican candidates on the field.

“I understand the anger Americans feel today,” Romney said, but added: “Here's what I know. Donald Trump is a phony, a fraud. His promises are as worthless as a degree from Trump University. 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 also claimed Trump, if nominated, would enable Hillary Clinton to be elected president.

The speech comes ahead of the Fox News Republican primary debate Thursday night at 9 p.m. ET in Detroit, the candidates' first showdown since Super Tuesday.

Trump swiftly responded to the ex-GOP nominee, taking to Twitter to claim he’s the “only one who can beat Hillary Clinton” and recalling Romney’s record in the presidential elections so far.

“I am not a Mitt Romney, who doesn't know how to win,” Trump tweeted.

He added: “Failed candidate Mitt Romney,who ran one of the worst races in presidential history,is working with the establishment to bury a big ‘R’ win!”

And speaking on MSNBC, Trump once again would not rule out mounting an independent bid.

Trump spokeswoman Katrina Pierson also countered Romney’s criticism of her candidate’s business record, pointing to Romney’s substantially smaller reported net worth.

“Mitt Romney has no right to say Donald Trump has failed in business,” she told Fox Business Network.

She predicted the American people will “rally” to Trump and said: “This is the establishment circling the wagons.”

The address Thursday amounted to an astonishing full-court press by Romney to stop Trump from winning the party’s nomination. Until now, Romney has mostly criticized Trump via Twitter and media interviews, but this is his most robust effort yet to rally the party and primary voters against the GOP front-runner.

Romney cast his alarm-bell warnings Thursday in dire terms, suggesting nothing less than the character of the nation is at stake, and saying Trump’s “brand of anger” has led “other nations into the abyss.” He said Trump “lacks the temperament to be president,” rebuking him for “bullying” and “greed” and “misogyny.” He borrowed a phrase from Marco Rubio, calling Trump a “con man” to boot.

On the sidelines, the party’s 2008 nominee also seemed to join with Romney in expressing concerns.

“I share the concerns about Donald Trump that my friend and former Republican nominee, Mitt Romney, described in his speech today,” Sen. John McCain, R-Ariz., said in a statement, urging voters to “think long and hard about who they want to be our next Commander-in-Chief and leader of the free world.”

2016 candidate Ohio Gov. John Kasich also gave Romney a Twitter thumbs-up, tweeting “well said” after the speech.

Romney has not yet endorsed a candidate in the race, or moved any closer publicly to reconsidering a run himself. Romney weighed another White House run at the beginning of 2015 before deciding against the move, reportedly after being out-raised by former Florida Gov. Jeb Bush -- who has since suspended his campaign.

Despite his decision not to run, there has been widespread media speculation that he could be tempted back into the race in the case of a contested Republican convention as a last gasp way to derail Trump.

The speech comes after Trump racked up a string of Super Tuesday wins, building a wide -- though not insurmountable -- delegate lead over closest rivals Ted Cruz and Marco Rubio.

The former Massachusetts governor has become increasingly vocal in his opposition to the business mogul.

Last week, Romney told Fox News’ Neil Cavuto that he believes there may be a “bombshell” in Trump’s tax returns, and called for Trump to release his returns as soon as possible. He reiterated that call on Thursday.

He later criticized Trump for not immediately disavowing the support of former Ku Klux Klan Grand Wizard David Duke.

Although Trump did later disavow Duke, Romney called his response “disqualifying and disgusting.”

Trump responded to Romney with barbs of his own, calling him "one of the dumbest and worst candidates in the history of Republican politics."