"His is not the temperament of a stable, thoughtful leader," Romney declared. He called Trump "a phony" who is "playing the American public for suckers," a man whose "imagination must not be married to real power."
Hours later, Trump lashed back, calling Romney "a choke artist" who lost to Barack Obama four years ago only because he was such a poor candidate.
In the most notable attacks on Trump as party leaders try to stop his run to the GOP nomination, Romney and House Speaker Paul Ryan, the party's vice presidential nominee in 2012, urged voters in the strongest terms to shun the former reality television star for the good of country and party.
The GOP's 2008 nominee, Arizona Sen. McCain, joined in, raising "many concerns about Mr. Trump's uninformed and indeed dangerous statements on national security issues that have been raised by 65 Republican defense and foreign policy leaders."
Romney embraced what might seem a long-shot approach to deny Trump the delegates necessary to secure the nomination. He did not call on Republicans to unify behind a single alternative candidate but outlined a plan to divide the electorate and force a contested national convention in July.
"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," Romney said.
As he spoke in Utah, Ryan said on Capitol Hill that "conservatism is being disfigured" by some of Trump's ideas and statements.
Underlying the remarkable criticism was a bleak reality for panicking Republican officials: Beyond harsh words, there is little they see they can do to stop Trump's march toward the Republican presidential nomination. Party leaders are poring over complicated delegate math, outlining their own hazy scenarios for a contested national convention and even flirting with the idea of a third-party effort.
Trump responded to Romney's speech at a campaign stop in Portland, Maine, saying the former Massachusetts governor "chickened out" when contemplation another presidential run this year when he understood he'd be going up against the billionaire businessman.
"He doesn't have what it takes to be president," Trump said, adding, "I made so much more money than Mitt."
Romney's views are irrelevant, he said. "Look, Mitt is a failed candidate."
The back-and-forth came as the Republican candidates prepared for their first post-Super Tuesday debate, scheduled for Thursday night in Detroit.
The pre-debate clash took place four years after Romney and Trump stood side by side in Las Vegas, with Trump saying it was a "real honor and privilege" to endorse Romney's White House bid. Romney at the time praised Trump's ability to "understand how our economy works and to create jobs for the American people."
On Thursday, Trump said Romney "was begging me" for an endorsement.
"I could have said, 'Mitt, drop to your knees.' He would have dropped to his knees," Trump said.
Earlier, in Utah, Romney assailed Trump's temperament, his business acumen and his ability to keep America safe.
"If we Republicans choose Donald Trump as our nominee, the prospects for a safe and prosperous future are greatly diminished," he said, warning that the kind of anger Trump has displayed has led other countries "into the abyss."
During his Capitol Hill press conference, Ryan dismissed comments Trump made earlier in the week that if the Wisconsin Republican didn't get along with him, Ryan would "pay a big price."
"I just laughed out loud," Ryan told reporters. "Sometimes, reality is stranger than fiction around here these days."
Voters have not so far responded to warning such as those of the Republican leaders on Thursday.
He padded his delegate lead with victories in seven Super Tuesday contests, with Cruz claiming three states and Florida Sen. Rubio picking up his first victory of the 2016 race.
Still, Trump is not yet on track to claim the nomination before the party's national gathering in July, according to an Associated Press delegate count. He has won 46 percent of the delegates awarded so far, and he would have to increase that to 51 percent in the remaining primaries.
March 15 could be last opportunity to stop Trump through the normal path of winning states and collecting delegates. A win for Rubio in his home state of Florida would raise questions about Trump's strength, as could a win for Kasich, Ohio's governor, on his home turf.
Kasich took aim at Trump on Thursday as well, saying for the first time that he needs to be stopped.
The GOP mayhem contrasts sharply with a clearer picture on the Democratic side, where Hillary Clinton is drawing broad support from voters and her party's leaders. Rival Sen. Bernie Sanders has vowed to keep up his fight, though his path to the nomination has become exceedingly narrow.
The Associated Press has asked Republican governors and senators if they would support Trump if he becomes the party's nominee. Of the 59 respondents, slightly fewer than half could not commit to backing him in November.
Peoples reported from Detroit. Associated Press writers Kathleen Ronayne in Detroit, David Sharp in Portland, Maine, and Julie Pace, Kathleen Hennessey, Andrew Taylor, Julie Bykowicz, Stephen Ohlemacher, Alan Fram, and Donna Cassata contributed to this report from Washington.