Donald Trump's nomination would hit hopes for safe and prosperous US - Romney
Donald Trump's nomination as presidential candidate would "greatly diminish" prospects for a safe and prosperous future for the United States, former Republican presidential nominee Mitt Romney said.
Mr Romney said the other Republican candidates would be better alternatives to the billionaire businessman, whom he called "a phony, a fraud".
The race for the Republican nomination, dominated by insults and name-calling, has seen Mr Trump's once-unlikely candidacy morph into an increasingly strong bid for his party's nomination for the November election.
"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," Mr Romney said of Mr Trump's rivals. "One of these men should be our nominee."
Mr Romney was relentless in his criticism, saying Mr Trump "is not the temperament of a stable, thoughtful leader. His imagination must not be married to real power."
Earlier on Thursday, Mr Trump dismissed Mr Romney as "a stiff" who "didn't know what he was doing" as the party's candidate in 2012. "People are energised by what I'm saying" and turning out in remarkable numbers to vote, Mr Trump told NBC.
The back-and-forth comes as Republican candidates prepared for the first post-Super Tuesday debate, scheduled for Thursday night.
Mr Trump is coming under increasing pressure from his party as he fights for the majority of delegates needed to win the nomination.
Mr Romney said a Trump nomination at the party's convention in July would enable Democrat Hillary Clinton to win the presidency.
Mr Romney also criticised Mrs Clinton, accusing her and her husband, former president Bill Clinton, of personally profiting from their positions of power.
Arizona senator John McCain, the losing Republican nominee in 2008, issued a statement endorsing Mr Romney's remarks. Mr Trump had dismissed Mr McCain's war-hero status for his long imprisonment during the Vietnam war.
Panicked Republican leaders say they still have options for preventing Mr Trump from winning the nomination, just not many good ones. They include a contested convention and even the long-shot prospect of a third party option.
Also on Thursday, dozens of conservative national security experts warned that Mr Trump is unfit to be commander in chief.
Former homeland security secretary Michael Chertoff and more than 70 others called Mr Trump's "embrace of the expansive use of torture" inexcusable. They also object to what they say is Mr Trump's "hateful, anti-Muslim rhetoric" and his advocacy for waging trade wars.
Despite Mr Trump's strong showing on Tuesday, he was not yet on track to claim the nomination before the party's national gathering, according to an Associated Press delegate count. He has won 46% of the delegates awarded so far, and he would have to increase that to 51% in the remaining primaries.
Mr Trump has 316 delegates so far, Mr Cruz 226 and Mr Rubio, 106. It takes 1,237 delegates to win the party nomination.
Party strategists cast March 15 as the last opportunity to stop Mr Trump through the normal path of winning states and collecting delegates. A win for Mr Rubio in his home state of Florida would raise questions about Mr Trump's strength, as could a win for Mr Kasich, Ohio's governor, on his home turf.
Retired neurosurgeon Ben Carson all but ended his bid on Wednesday, saying he would skip the debate and declaring he did "not see a political path forward."
On the Democratic side, Mrs Clinton was drawing broad support from voters and her party's leaders. Rival Bernie Sanders vowed to keep up the fight, though his path to the nomination has narrowed. So far, Mrs Clinton has at least 1,005 delegates and Mr Sanders 373. It takes 2,383 Democratic delegates to win.
In response at a campaign speech in Maine, Mr Trump made much of his own endorsement of the former Massachusetts governor in his failed race to unseat President Barack Obama in the 2012 contest.
Mr Trump said Mr Romney begged for his backing and he could have "said, 'Mitt, drop to your knees.' He would have dropped to his knees".