Mitt Romney is set to lash out at Donald Trump in a bid to dislodge him from the top of the Republican party's presidential nominating race, branding the billionaire real-estate developer as untrustworthy and saying he'd be a boon to Democratic candidate Hillary Clinton.

"Donald Trump is a phony, a fraud," Romney, the 2012 Republican nominee for president, will say later on Thursday to the Hinckley Institute of Politics at the University of Utah according to a transcript provided to Bloomberg News by a person familiar with his prepared remarks. "He's playing the American public for suckers."

The criticism marks the bluntest attempt so far by the Republican establishment to slow Trump's momentum after his victories on Tuesday in the single biggest day of voting in the Republican race. Of 15 U.S. states that have held nominating contests to date, Trump has won 10 and has 46 percent of the delegates awarded so far.

Trump responds on Twitter

Trump took to Twitter to respond Thursday: "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!"

The message to his social-media followers came after Trump had already taken on Romney Wednesday night after the planned speech became public. "Looks like two-time failed candidate Mitt Romney is going to be telling Republicans how to get elected. Not a good messenger!"

'Policies would lead to recession'

Romney, a former governor of Massachusetts, will say that Trump as president would damage the country at home and abroad.

"His domestic policies would lead to recession," Romney will say. "His foreign policies would make America and the world less safe. He has neither the temperament nor the judgment to be president."

As Hillary Clinton moves closer to becoming the Democratic nominee for November's presidential election, Romney will say that any decision to make Trump the Republican counterpart would bolster Clinton's chances of succeeding President Barack Obama.

"A person so untrustworthy and dishonest as Hillary Clinton must not become president," Romney is due to say. "But a Trump nomination enables her victory."

To stop a runaway train

Romney's speech is the latest attempt by long-time Republicans to stop his momentum after Trump's competitors in the race for the party's nomination — from former Florida governor Jeb Bush to Sens. Ted Cruz of Texas and Marco Rubio of Florida — have been unable to collect more than a handful of wins at the polls.

Trump has managed to collect a few endorsements, including New Jersey Gov. Chris Christie, Maine Gov. Paul LePage, and Sen. Jeff Sessions of Alabama.

After Trump's Super Tuesday wins this week, a pair of super PACs that oppose him stepped up their advertising efforts as Republicans wrestled with whether and how to stop Trump at this late stage of the nominating process — and which alternative candidate to unite behind. The ads targeted the billionaire's Trump University as a "scam" duping "victims."

Willingness to back Trump

At least some party leaders, however, began showing a willingness to back Trump.

"I like the fact that he knows how to negotiate. We've had seven, eight years of somebody who doesn't know how to negotiate," said Sen. Ron Johnson of Wisconsin, who faces a challenging re-election bid in a state won twice by Barack Obama.

Romney has previously questioned Trump's reluctance to condemn white supremacist David Duke in an interview, although Trump had done so days before, and his failure to release his tax returns to the public, saying that he suspected Trump was hiding a "bombshell."

Trump responded that Romney had sought his endorsement when running for president in 2012, at which time Romney praised Trump business acumen and success.