Former Republican presidential candidate MittREUTERS

Former Republican 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 Republican unease with the incendiary New York billionaire.

Billed as a major speech by the 2012 nominee, Romney's appearance in Utah comes on the day that Trump and his three remaining 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 that gave extra momentum to Trump but did not knock out his rivals.

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Mainstream figures in the party are seeking a strategy 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 are critical of 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 for the first time since last year by Fox News anchor Megyn Kelly, who angered him with her questioning at the Republicans' first debate on Aug. 6, prompting him to cancel participation in a debate in Iowa in January, a move that appeared to cost him some votes.

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 become increasingly disturbed at the prospect of Trump becoming the party's nominee.

They said he would specifically cite Trump's refusal to release his tax returns and his initial reluctance to disavow an endorsement from a former leader of the Ku Klux Klan white supremacist group.

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Romney has been leaning toward endorsing Rubio, said two sources close to the former Massachusetts governor. Sources said he was not expected to endorse anyone in his speech.

One senior Republican source familiar with his thinking said Romney had debated privately whether his endorsement would help, since pro-Romney voters had already gotten behind Rubio, 44, a U.S. senator from Florida.

In his speech, Romney is expected to single out for praise both Rubio, and Cruz, 45, a U.S. senator from Texas, and possibly Kasich, 63, the Ohio governor.

Trump has consistently targeted Romney for criticism throughout his run, saying he should have defeated Obama in 2012 but that Romney made mistakes that killed his chances.

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 has largely steered clear of the anti-Trump effort and tried to remain above the fray.

Rubio went on the attack against Trump at the last debate on Feb. 25 and has attempted to establish himself as the main Trump alternative by labeling him as a "con artist" who has escaped serious vetting by a news media fixated on his star power and brash rhetoric.

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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."

Trump's campaign on Monday reached out to House of Representatives Speaker Paul Ryan, the country's top elected Republican, after days of criticism of Trump from Ryan. At a news conference on Tuesday, Trump said Ryan could either get along with him or "pay a price."

"We have heard from the campaign, but the two have not yet spoken," said Ryan spokesman Brendan Buck. "We expect the speaker to be in touch with all the remaining candidates soon to discuss our efforts to build a bold conservative policy agenda for 2017."

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