WATCH: Mitt Romney portrays Trump as a failure, buffoon and con-man in scathing speech

Former U.S. presidential candidate Mitt Romney gave a blistering rebuke of 2016 Republican front-runner Donald Trump on Thursday, leading an attempt by the party establishment to halt the rise of the outspoken New York billionaire.

Romney, a Republican elder statesman and the nominee four years ago, urged Republicans in states that have not yet held nominating contests to back Trump's opponents to stop his march to the nomination for the Nov. 8 election to succeed President Barack Obama.

"Here's what I know. Donald Trump is a phony, a fraud," said Romney, 68, in a hard-hitting 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," he said.

Trump has made his party's mainstream uneasy with his positions on trade and immigration, including his calls to build a wall between the United States and Mexico, deport 11 million illegal immigrants and temporarily bar Muslims from entering the country.

But Romney's strategy risks backfiring by further energizing Trump's supporters, who are angry with a party they see as not representing their views on illegal immigration, trade and America's role in the world.

"If you’re Trump, this is like getting the good kind of Kryptonite," Republican strategist Doug Heye said.

The real estate mogul dismissed Romney in television interviews and posts on Twitter, calling him "a failed candidate" who had "begged" him for an endorsement in 2012 when he eventually lost to Obama.

"Mitt Romney is a stiff," Trump told NBC's "Today" program.

In a speech he wrote himself, Romney said Trump's economic policy would sink America "into prolonged recession" and slammed his business acumen and temperament.

He warned that polls show Trump would likely lose to possible Democratic presidential candidate Hillary Clinton in November.

Earlier, more than 70 Republican national security leaders signed a scathing open letter opposing Trump and his stance on many foreign policy issues.

Senator John McCain, the Republican presidential nominee in 2008 who has sparred with Trump, joined the wave of criticism of the billionaire who took a step toward securing the nomination when he won most of the Republican contests on Super Tuesday this week.

"DANGEROUS"

"I would also echo the many concerns about Mr. Trump’s uninformed and indeed dangerous statements on national security issues," said McCain, chairman of the Senate Armed Services Committee.

Romney pointed to Trump's refusal to release his tax returns and initial reluctance to disavow an endorsement from a former leader of the Ku Klux Klan white supremacist group.

Romney, who did not endorse anyone, suggested Republicans vote for candidates with the best chance against Trump in states still to hold nominating votes: Senator Marco Rubio in Florida, Ohio Governor Kasich in his home state, and Senator Ted Cruz from Texas where he is strong.

Romney's speech came hours before Trump and his rivals share a stage in Detroit at 9 p.m. EST for a debate hosted by Fox News.

(Additional reporting by Susan Heavey, Megan Cassella, Eric Beech, James Oliphant; Writing by Steve Holland and Roberta Rampton; Editing by Lisa Von Ahn and Alistair Bell)

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