Romney, an elder statesman in the party, urged Republicans in states that have not yet held nominating contests to vote for Trump’s opponents to stop his march to the nomination for the November 8 election to succeed President Barack Obama.

“Here’s what I know. Donald Trump is a phony, a fraud,” said Romney, 68, who has kept a low profile since losing to 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 on Thursday.

Trump has made his party’s establishment 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.

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.

Romney, who did not endorse anyone, suggested Republicans vote for candidates who appear poised to do the best 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 said Trump’s economic policy would sink America “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, Trump dismissed 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,” Trump told NBC’s “Today” program.

Romney’s strategy risks backfiring by further energising 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.

Romney’s speech came hours before Trump and rivals Marco Rubio, Ted Cruz and John Kasich share a debate stage in Detroit.

Reuters