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Former GOP presidential nominee Mitt Romney battled Republicans' current front-runner Donald Trump on Thursday — both delivering a blistering series of insults against each other's temperaments, electability and business acumen.
Romney's attacks coincided with the several current and former GOP leaders launching a last-ditch effort to hinder Trump's assent to their party's coronation.
"Let me put it plainly, if we Republicans choose Donald Trump as our nominee, the prospects for a safe and prosperous future are greatly diminished," Romney said.
Later on Thursday, Trump responded to that criticism, saying that Romney had "failed badly" as a candidate, and that he had been disloyal after Trump's 2012 endorsement of him. "You can see how loyal he is: He was begging for my endorsement," Trump said. "I could have said 'Mitt drop to your knees,' he would've dropped to his knees."
Instead, Trump said he found Romney's Thursday comments "very nasty — I mean, I thought he was a better person than that."
Billed as a major speech by the 2012 nominee, Romney's appearance in Utah came hours before Trump and rivals Marco Rubio,Ted Cruz and John Kasich share a debate stage in Detroit. In that address, Romney attacked Trump's proposals, his temperament and his adherence to the truth.
"There are a number of people who claim that Mr. Trump is a con man, a fake — thank you," Romney said. "There's plenty of evidence that Mr. Trump is a con man, a fake. Mr. Trump has changed his positions not just over the years, but over the course of the campaign, and on the Ku Klux Klan, daily for three days in a row."
Romney acknowledged that many look at Trump's lead in GOP delegates, and believe his victory is all but assured, but the former Massachusetts governor said the Republicans can find another way.
"If the other candidates can find common ground, I believe we can nominate a person who can win the general election and who will represent the values and policies of conservatism," he said. "Given the current delegate selection process, this means that I would vote for Marco Rubio in Florida, for John Kasich in Ohio, and for Ted Cruz or whichever one of the other two contenders has the best chance of beating Mr. Trump in a given state."
In arguing against the Republican front-runner, Romney warned Americans about the future, saying that Trump's "is the very brand of anger that has led other nations into the abyss."
"His domestic policies would lead to recession. His foreign policies would make America and the world less safe. He has neither the temperament nor the judgment to be president," Romney said. "And his personal qualities would mean that America would cease to be a shining city on a hill."
Trump implied Thursday that Romney's criticism was because he wished he was the one running for president. In fact, Trump said he's the reason that "choke artist" Romney decided to stay out of this year's race.
Laying into the New York businessman's proposals, Romney said he had concluded Trump is "very, very not smart" when it comes to foreign policy, and his economic plans would sink the country "into a prolonged recession."
In fact, Romney attacked Trump's business acumen, highlighting past business bankruptcies and failed ventures.
In addition to Romney's comments, his former vice-presidential candidate, House Speaker Paul Ryan, said he would speak out in the presidential race if he saw "conservatism being disfigured" in campaign rhetoric.
Republicans' 2008 presidential nominee, U.S. Sen. John McCain of Arizona, said Thursday that he shares Romney's concerns about Trump.
In another set of Thursday comments, New Jersey Gov. Chris Christie sought to publicly explain his endorsement of Trump.
"As a Republican I feel strongly about making sure that [Democratic front-runner] Hillary Clinton does not become president of the United states, and I believe Donald Trump is the best person — of those remaining — to do that," he said. "I've made a choice: Some people agree with that choice, some people disagree with that choice. That's what political choices are all about."
"I understand the things that Gov. Romney is saying, and he's objected to. That's his opinion, and I respect him as I always have, and he's welcome to his view. I just happen to disagree with the conclusion that he comes to," Christie said.
Christie emphasized that he is not "a full-time surrogate for Donald Trump," adding that he does not have any scheduled plans to hit the campaign trail again with him, but he's "sure" he will eventually. Addressing the proliferation of social media chatter about his appearance behind Trump during Super Tuesday, Christie said the "armchair psychiatrists should give it a break."
"It wasn't the kind of circumstance where I'd be jumping up and down cheering and smiling: He was answering questions from the national press corps and I was listening," Christie said. "This is part of the hysteria of the people who oppose my Trump endorsement — they want to read anything into it that can be negative."
The 9 p.m. EST (0200 GMT Friday) debate hosted by Fox News will be the candidates' first face-to-face gathering since Super Tuesday nominating contests this week gave extra momentum to Trump but did not knock out his rivals.
Mainstream figures in the party are trying to halt the real estate mogul's march to the nomination for the Nov. 8 election to succeed Democratic President Barack Obama.
Some party leaders and donors have criticized Trump's positions on trade and immigration, including his calls to build a wall between the United States and Mexico, deport 11 million illegal immigrants and temporarily bar Muslims from entering the country.
At the debate, Trump, 69, will be questioned by Fox News anchor Megyn Kelly for the first time since last year. Her questions angered him at the Republicans' first debate on Aug. 6, prompting him to withdraw from one in Iowa in January, a move that appeared to cost him some votes.
The Detroit debate will be one more opportunity for Rubio and Cruz to try to slow Trump's momentum. They are the last two anti-Trump candidates standing in what has been a bruising nomination battle. Kasich, endorsed by the Detroit News on Thursday, has largely steered clear of the anti-Trump effort and tried to remain above the fray.
—Reuters contributed to this report.