DETROIT (Reuters) - Former Republican U.S. presidential nominee Mitt Romney plans to deliver a rebuke of 2016 party front-runner Donald Trump on Thursday in a high-profile display of establishment unease with the incendiary New York billionaire.
Billed as a major speech by the 2012 nominee, Romney's appearance in Utah comes hours before Trump and rivals Marco Rubio, Ted Cruz and John Kasich share a debate stage in Detroit.
The 9 p.m. EST debate hosted by Fox News will be the candidates' first face-to-face gathering since Super Tuesday nominating contests this week gave extra momentum to Trump but did not knock out his rivals.
Mainstream figures in the party are trying to halt the real estate mogul's march to the nomination for the Nov. 8 election to succeed Democratic President Barack Obama.
Some party leaders and donors have criticized Trump's 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.
At the debate, Trump, 69, will be questioned by Fox News anchor Megyn Kelly for the first time since last year. Her questions angered him at the Republicans' first debate on Aug. 6 and prompting him to withdraw from one in Iowa in January, a move that appeared to cost him some votes.
"I'm ready; I'm absolutely ready," he told ABC's "Good Morning America" program on Thursday when asked if he was prepared to face Kelly again.
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.
Sources familiar with his thinking said Romney, in private conservations with friends and allies, had expressed increasing concern that Trump could become the party's nominee.
In a speech at 9:30 a.m. MST in Salt Lake City, Romney plans to call Trump "a phony, a fraud" who is "playing the American public for suckers," according to media reports citing excerpts from his remarks.
Romney will point to Trump'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, sources said.
Still, Romney is not expected to endorse anyone in his speech. Two sources close to the former Massachusetts governor said he had been leaning toward backing Rubio, the 44-year-old U.S. senator from Florida who won the Miami Herald's endorsement late Wednesday.
Romney will point to Rubio; Cruz, a 45-year-old U.S. senator from Texas; and Ohio Governor Kasich, 63, as offering serious policy alternatives. He also will call Democratic presidential front-runner Hillary Clinton "untrustworthy and dishonest," according to the excerpts.
On Thursday, Trump, in a round of television interviews and posts on Twitter, called Romney "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.
SPARKS COULD FLY
The Detroit debate will be one more opportunity for Rubio and Cruz to try to slow Trump's momentum. They are the last two anti-Trump candidates standing in what has been a bruising nomination battle. Kasich, endorsed by the Detroit News on Thursday, has largely steered clear of the anti-Trump effort and tried to remain above the fray.
Rubio attacked Trump at the last debate on Feb. 25 and has attempted to establish himself as the party's main alternative by labeling him as a "con artist" who has escaped serious vetting by news media fixated on his star power and brash rhetoric.
Asked if he would try to appear more presidential at the forum, Trump rejected that idea, telling NBC he expected to be roundly attacked and has to hit back.
He also dismissed a scathing open letter from more than 60 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.
"We commit ourselves to working energetically to prevent the election of someone so utterly unfitted to the office," the letter on Wednesday read.
Veteran Republican foreign policy strategist Richard Grenell said the debate at Detroit's Fox Theater could be a lively one.
"Since Trump is clearly on a roll, he needs to pivot and start uniting the Republican Party and alleviate the concerns that he can't act presidential," Grenell said. "Rubio and Cruz are going to need to take each other out in order to whittle down the field."
Meanwhile, Trump has also tangled with House of Representatives Speaker Paul Ryan. After criticism from the nation's top elected Republican, Trump said Ryan could either get along with him or "pay a price."
Ryan told Fox's "Fox & Friends" program on Thursday that he was making arrangements to talk with all the remaining candidates soon but that he had not spoken with Trump and was refraining from endorsing anyone.
"I just don't know the Donald," Ryan said.
(Reporting by Steve Holland; Additional reporting by Susan Heavey and Megan Cassella; Editing by Peter Cooney and Lisa Von Ahn)
This article was funded in part by SAP. It was independently created by the Reuters editorial staff. SAP had no editorial involvement in its creation or production.