US Republican Mitt Romney has delivered a scathing critique of Donald Trump, the party's 2016 presidential front runner, saying "improvident choices" could dim the US's future.
"If we make the right choices, America's future will be even better than our past and better than our present," Mr Romney said in a speech at the University of Utah in Salt Lake City.
"On the other hand, if we make improvident choices, the bright horizon I've described will not materialise."
"Let me put it very plainly, if we Republicans choose Donald Trump as our nominee, the prospects for a safe and prosperous future are greatly diminished," Mr Romney added.
"Here's what I know. Donald Trump is a phony, a fraud," said Mr Romney who has kept a low profile since losing to Barack Obama in 2012.
"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.
Mr Trump has made his party's establishment is uneasy with Trump's positions on trade and immigration, including his calls to build a wall between the United States and Mexico, deporting 11 million illegal immigrants and temporarily barring Muslims from entering the country.
Mr Romney pointed to the billionaire real-estate developer'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.
Mr Romney, who did not endorse anyone, suggested Republicans vote for candidates who appear poised to do the best against Mr 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.
Mr Romney said Mr Trump's economic policy would sink the US "into prolonged recession" and his foreign policy would endanger the country. He criticised his business acumen and his temperament.
"This is the very brand of anger that has led other nations into the abyss," he said.
Ahead of the speech, much of which had been previewed in excerpts, Mr Trump dismissed Mr Romney in television interviews and posts on Twitter, calling him "a failed candidate" who had "begged" him for an endorsement in 2012.
"Mitt Romney is a stiff," Mr Trump said.
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!— Donald J. Trump (@realDonaldTrump) March 3, 2016
Mr Romney's strategy risks backfiring by further energising Mr Trump's supporters, who are angry with a party they see as not defending their interests.
"If you're Trump, this is like getting the good kind of Kryptonite," Republican strategist Doug Heye said.
Mr Romney's speech came hours before Mr Trump and his rivals share a debate stage in Detroit.
The debate will be the candidates' first face-to-face gathering since the Super Tuesday nominating contests this week gave extra momentum to Mr Trump but did not knock out his rivals.
US Senator John McCain, the Republican presidential nominee in 2008, joined Mr Romney in criticising Mr Trump, particularly on foreign policy.
"I share the concerns about Donald Trump that my friend and former Republican nominee, Mitt Romney, described in his speech today," Mr McCain said in a statement.
"I would also echo the many concerns about Mr Trump's uninformed and indeed dangerous statements on national security issues that have been raised by 65 Republican defense and foreign policy leaders," he said of the letter, published online.
Mr McCain, chairman of the Senate Armed Services Committee, said at a time of global turmoil: "I want Republican voters to pay close attention to what our party's most respected and knowledgeable leaders and national security experts are saying about Mr Trump."