The 2012 Republican nominee for the US presidency has urged his own party to oppose the nomination of Donald Trump this year, warning that the current frontrunner would "greatly diminish" prospects for a safe and prosperous future for the country.
In a stinging speech on Thursday, Mitt Romney said 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 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 said Trump does not possess "the temperament of a stable, thoughtful leader. His imagination must not be married to real power."
Earlier on Thursday, Trump dismissed Romney as "a stiff" who "didn't know what he was doing" as the party's candidate in 2012. During the same year, Trump endorsed Romney as president.
"People are energised by what I'm saying" and turning out in remarkable numbers to vote, Trump told the NBC television.
Al Jazeera's Patty Culhane, reporting from Washington DC, said there is a concerted effort by party leaders to stop Trump.
She said that prominent members of the party are also planning to run television ads against the New York businessman.
"We haven't seen anthing like this before," she said, referring to the party trying to take down its own nominee. "But we don't know if it's going to work."
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.
Arizona Senator 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.
Also on 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 percent of the delegates awarded so far, and he would have to increase that to 51 percent in the remaining primaries 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.
Source: Al Jazeera and agencies