Mitt Romney, the Republican US presidential nominee in 2012 and former governor of Massachusetts, on Thursday lashed out at the party's front-runner Donald Trump, describing the real estate mogul as "a phony, a fraud."

"Here's what I know. Donald Trump is a phony, a fraud," the 68-year-old Romney said during a speech.

"He's playing the members of the American public for suckers. He gets a free ride to the White House and all we get is a lousy hat," Romney added, referring to Trump's notorious campaign hats with his slogan: "Make America great again."

The former governor said Trump's economic policy would be detrimental to the country.

"If Donald Trump's plans were ever implemented, the country would sink into a [prolonged] recession," Romney noted.

"Even as Donald Trump has offered very few specific economic plans, what little he has said is enough to know that he would be very bad for American workers and for American families," he added.

'Fundamentally dishonest'

US Senator John McCain, who was the Republican presidential nominee in 2008, said he backed Romney's comments and questioned Trump's policy proposals.

Romney's speech lambasting Trump marks a new chapter in the Republican leadership's attempts to curb the businessman's unorthodox appeal after taking the lead during the conservative party's latest primaries.

Late Wednesday, more than 70 conservative national security leaders and experts, including former Homeland Security Secretary Michael Chertoff, published an open letter asserting their opposition to a Trump presidency, calling him "fundamentally dishonest."

Republicans should "think long and hard about who they want to be our next commander-in-chief and leader of the free world," the letter added.

Four years later, Romney backtracks

Romney's speech on Thursday is a stark contrast to his positive description of the billionaire in 2012 after Trump endorsed him for the presidential election.

"Donald Trump has shown an extraordinary ability to understand how our economy works - to create jobs for the American people. He's done it here in Nevada. He's done it across the country," Romney said in February 2012.

Meanwhile, Trump fired back at the former governor after his Thursday speech, calling him a "failed candidate."

"Why did Mitt Romney beg me for my endorsement four years ago?" Trump said in a tweet.

"Failed candidate Mitt Romney, who ran one of the worst races in presidential history, is working with the establishment to bury a big 'R' win," the billionaire added, referring to the Republican party by its first letter.

At a rally in New Hampshire, Trump defended his economic policy, saying: "No one knows more about trade than me."

Trump is expected to debate Texas Senator Ted Cruz, Florida Senator Marco Rubio and Ohio's Governor John Kasich on Thursday night in Detroit.

ls/sms (Reuters, AP, AFP)