The former Republican nominee has come out guns blazing for the front-runner
Former Republican presidential nominee Mitt Romney has labelled the party's 2016 White House front runner Donald Trump a "phony" in an astonishing attack.
In a speech at the University of Utah on Thursday, the former Massachusetts Governor urged Republicans to shun the real estate billionaire for the good of the party.
As a growing group of anxious Republican leaders attempt to halt Mr Trump's march to their party's nomination, Mr Romney launched a deeply personal attack.
"Here's what I know. Donald Trump is a phony, a fraud.His promises are as worthless as a degree from Trump university. He's playing the American public for suckers - he gets a free ride to the White House and all we get is a lousy hat."
Mr Romney - who is not running for the White House this time - also sought to undermine Mr Trump's success story.
"A business genius he is not," he said.
He said a win for Mr Trump at the Republican convention in Cleveland in July would gift Democrat Hillary Clinton the keys to the White House.
Mr Romney said Mr Trump, who has courted controversy with remarks against women, Muslims and Mexicans, "has neither the temperament nor the judgement to be president".
He said the only serious policy proposals have come from the other Republican candidates, Ted Cruz, Marco Rubio, and John Kasich.
Mr Romney's speech also claimed Mr Trump's "domestic policies would lead to recession".
He said Mr Trump's "foreign policies would make America and the world less safe" and his "personal qualities would mean that America would cease to be a shining city on a hill".
On Thursday, more than 70 national security experts - including former Homeland Security Secretary Michael Chertoff - said Mr Trump was unfit to be commander in chief.
Mr Trump enjoyed victories in seven states on Super Tuesday, with Texas Senator Mr Cruz claiming three states and Florida Senator Mr Rubio winning just one.
In the primary race so far, he has 316 delegates so far, Mr Cruz has 226 and Mr Rubio, 106. It takes 1,237 delegates to win the party nomination in July.
Party strategists have cast 15 March 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 could raise questions over Mr Trump's strength.
The candidates will have an opportunity to make their case to voters during a prime-time debate on Thursday night.