WASHINGTON — Donald Trump's nomination as the Republican presidential candidate would "greatly diminish" prospects for a safe and prosperous future for the United States, former Republican presidential nominee Mitt Romney said in a stinging speech on Thursday.
Romney said the other Republican candidates would be better alternatives to the billionaire businessman, whom he called "a phoney, a fraud."
It is virtually unheard of for a former party standard-bearer to attack a fellow party member and candidate for the presidential nomination. But it has been a political season like none before.
The Republican race, already dominated by insults and name-calling, has seen 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," Romney said of Trump's rivals. "One of these men should be our nominee."
Romney was relentless in his criticism, saying Trump "is not the temperament of a stable, thoughtful leader. His imagination must not be married to real power."
In response at a campaign speech in Maine, 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.
Trump said Romney begged for his backing and he could have "said, 'Mitt, drop to your knees.' He would have dropped to his knees."
Earlier in the day, Trump dismissed Romney as "a stiff" who "didn't know what he was doing" as the party's candidate in 2012. "People are energized by what I'm saying" and turning out in remarkable numbers to vote, Trump told NBC.
The back-and-forth comes as Republican candidates prepared for the first post-Super Tuesday debate, scheduled for Thursday night.
Trump is coming under increasing pressure from his party as he fights for the majority of delegates needed to win the nomination.
Romney said a Trump nomination at the party's convention in July would enable Democrat Hillary Clinton to win the presidency.
Romney also criticized Clinton, accusing her and her husband, former President Bill Clinton, of personally profiting from their positions of power.
Arizona Sen. John McCain, the losing Republican nominee in 2008, issued a statement endorsing Romney's remarks. Trump had dismissed McCain's war-hero status for his long imprisonment during the Vietnam war.
Panicked Republican leaders say they still have options for preventing 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 Thursday, dozens of conservative national security experts warned that Trump is unfit to be commander in chief.
Former Homeland Security Secretary Michael Chertoff and more than 70 others called Trump's "embrace of the expansive use of torture" inexcusable. They also object to what they say is Trump's "hateful, anti-Muslim rhetoric" and his advocacy for waging trade wars.
Despite 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 per cent of the delegates awarded so far, and he would have to increase that to 51 per cent in the remaining primaries.
Trump has 316 delegates so far, Texas Sen. Cruz 226 and Florida Sen. Rubio, 106. It takes 1,237 delegates to win the party nomination.
Party 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.
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."
On the Democratic side, 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, Clinton has at least 1,005 delegates, Sanders 373. It takes 2,383 Democratic delegates to win.