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Speaking at the University of Utah, Romney warned a packed auditorium that any of the other GOP candidates would be a better alternative to the billionaire businessman.
"The only serious policy proposals that deal with the broad range of national challenges we confront today, come from Ted Cruz, Marco Rubio, and John Kasich," Romney said of Trump's GOP rivals. "One of these men should be our nominee."
Romney is charging into the increasingly divisive White House race with a verbal lashing of Donald Trump and a plea for fellow Republicans to shun the front-runner for the good of country and party.
"His is not the temperament of a stable, thoughtful leader. His imagination must not be married to real power," Romney said.
Romney branded Trump as "a phony, a fraud" whose "promises are as worthless as a degree from Trump University" in his speech.
U.S. Senator John McCain, the Republican presidential nominee in 2008, joined Romneyin criticizing Trump, particularly on foreign policy.
"I share the concerns about Donald Trump that my friend and former Republican nominee, Mitt Romney, described in his speech today," McCain said in a statement.
"I would also echo the 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," he said of the letter, published online.
McCain, chairman of the Senate Armed Services Committee, said at a time of global turmoil, "I want Republican voters to pay close attention to what our party's most respected and knowledgeable leaders and national security experts are saying about Mr. Trump."
Earlier Thursday, Trump dismissed Romney as "a stiff" who "didn't know what he was doing" as the party's candidate in 2012 and blew a chance to beat President Barack Obama. "People are energized by what I'm saying" in the campaign and turning out in remarkable numbers to vote, Trump told NBC's "Today."
The back-and-forth comes as Republican candidates prepared for the first post-Super Tuesday debate, scheduled for Thursday in Detroit, with Trump coming under increasing pressure from his party as he fights for the majority of delegates needed to win the nomination.
Thursday's condemnation and counter was coming four years after the two men stood side by side in Las Vegas, with Trump saying it was a "real honor and privilege" to endorse Romney's White House bid. Accepting, Romney said it was a "delight" to have Trump on his side and praised him for ability to "understand how our economy works and to create jobs for the American people."
Panicked GOP leaders say they still have options for preventing Trump from winning the GOP nomination — just not many good ones.
Romney also said that a Trump nomination at the party's convention in Cleveland in July would enable Democrat Hillary Clinton to win the presidency. He contended that Trump "has neither the temperament nor the judgment to be president."
Romney's involvement comes as party elites pore over complicated delegate math, outlining hazy scenarios for a contested convention and even flirting with the long-shot prospect of a third party option.
Giving Romney the back of his hand, Trump turned his sights on the general election. His campaign reached out to House Speaker Paul Ryan's office to arrange a conversation between the two men, and urged Republican leaders to view his candidacy as a chance to expand the party.
Trump padded his lead with victories in seven Super Tuesday contests, with Texas Sen. Ted Cruz claiming three states and Florida Sen. Marco Rubio picking up his first victory of the 2016 race.
Despite Trump's strong night, he was 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.
GOP strategists cast March 15 as the 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.
The candidates have a high-profile opportunity to make their case to voters in Thursday's Fox News debate. Retired neurosurgeon Ben Carson all but ended his bid Wednesday, saying he would skip the debate and declaring he did "not see a political path forward."
The GOP mayhem contrasted sharply with a clearer picture on the Democratic side, where Hillary Clinton was drawing broad support from voters and her party's leaders. Rival Sen. Bernie Sanders vowed to keep up the fight, though his path to the nomination has become exceedingly narrow.
Romney argues that Trump's "domestic policies would lead to recession. His foreign policies would make America and the world less safe," Romney says. "And his personal qualities would mean that America would cease to be a shining city on a hill."
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.
One long-shot idea rumbling through power corridors in Washington was the prospect of a late third-party candidate to represent more mainstream conservatives. Former Texas Gov. Rick Perry has been approached by "a mixture of people" about being part of a third-party bid, according to Jeff Miller, who managed Perry's failed GOP presidential campaign. But Miller said Perry found the idea "ludicrous."