Former Massachusetts governor Mitt Romney, the Republican Party's presidential nominee who lost to President Barack Obama in 2012, entered the political arena once again to denounce Donald Trump, the heavy favourite to win the GOP nomination, calling him a "fraud" who is just fooling the American people.

"Dishonesty is Trump's hallmark," Romney told an audience at the University of Utah in Salt Lake City on Thursday, WND reported.

"Let me put it plainly, if we Republicans choose Donald Trump as our nominee, the prospects for a safe and prosperous future are greatly diminished," he said.

But even before Romney could deliver his speech, Trump—who apparently received a copy of the speech much earlier—delivered his own blows via Twitter at the former Massachusetts governor on Wednesday night, Newsmax reported.

Trump blasted the 2012 GOP presidential nominee as a "two-time failed candidate," calling him "not a good messenger." Romney lost in the 2008 Republican presidential race and in the 2012 faceoff against the reelectionist Democrat President Barack Obama.

The brash New York billionaire noted that Romney is going after him now after seeking his endorsement four years ago for his failed White House bid where he ran "one of the worst races in presidential history."

Trump adviser Dan Scavino followed up the Romney attack shortly after the speech was delivered live, thanking Romney for the "endorsement."

Savino declared the GOP establishment attack was "gold," and making the candidate "stronger."

Romney slammed Trump's economic plan, saying it would "sink the country into depression," launch a trade war, and "balloon the deficit."

He bluntly called Trump a fraud, saying he is not the "huge business success" that some people think. "His bankruptcies have crushed small businesses and the men and women who worked for them," he said.

Romney referred to Trump Airlines, Trump University, Trump Steaks and a host of other Trump business ventures, and asked: Where are they now?

"A business genius, he is not," he said.

Romney then endorsed any of the three remaining GOP contenders—Senators Marco Rubio and Ted Cruz, and Gov. John Kasich—although he said at the start of his speech that he was not supporting a candidate.

He blasted Trump's foreign-policy statements, which he called "bombast" that was "alarming our allies." He said the billionaire's "insulting [of] all Muslims will keep many of them from engaging with us in their fight against terrorists."

Romney then smacked Trump as lacking the proper "temperament to be president," calling him untrustworthy.

He hurled other slurs at Trump, calling the New York billionaire a "bully," misogynist, accusing him of "absurd third-grade theatrics," and adding: "Imagine your children and your grandchildren acting the way he does."

Romney then said "a Trump nomination enables" Hillary Clinton to take over the White House.

Romney concluded his speech by saying: "Here's what I know. Donald Trump is a phony, a fraud, his promises are as worthless as a degree from Trump University. 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. His domestic policies would lead to recession. His foreign policies would make America and the world less safe. He has neither the temperament nor the judgment to be president. And his personal qualities would mean that America would cease to be a shining city on a hill."

His final statement: "This is a time of choosing. God bless us to choose a nominee who would make that vision a reality."