The speech from Romney, who sees himself as an elder statesman in the party, comes hours before Trump and rivals Marco Rubio, Ted Cruz and John Kasich share a debate stage in Detroit.

"Think of Donald Trump's personal qualities, the bullying, the greed, the showing off, the misogyny, the absurd third grade theatrics," Romney said. "We have long referred to him as "The Donald." He is the only person in America to whom we have added an article before his name. It wasn't because he had attributes we admired. "

He also said Trump would not lead the Republican party to victory in the presidential race, "Trump relishes any poll that reflects what he thinks of himself. But polls are also saying that he will lose to Hillary Clinton."

He also called on Trump to release his tax returns, "We will only really know if he is the real deal or a phony if he releases his tax returns and the tape of his interview with the New York Times," Romney said. "I predict that there are more bombshells in his tax returns."

"Attacking me as he surely will won't prove him any less of a phony," Romney added.

Trump dismissed Romney in a round of television interviews and posts on Twitter, calling him "a failed candidate" who had "begged" him for an endorsement in 2012 before losing to Obama.

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

Romney, 68, has kept a low profile since losing to Obama in 2012. He flirted with a 2016 campaign but ultimately decided the country needed fresh leaders.

He decided to give the speech on his own, and was not asked to do so by the Republican National Committee, an RNC official said.

His speech follows a scathing open letter signed by more than 70 Republican national security leaders who said they opposed Trump and his stance on many foreign policy issues. His presidency would make the United States less safe and diminish its world standing, they said