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A divided Republican Party erupted into open and bitter warfare on Thursday as its two previous presidential nominees delivered an extraordinary rebuke of its current front-runner, Donald J. Trump, warning that his election could put the United States and its democratic system in peril.

In a detailed, thorough and lacerating assault on Mr. Trump and the angry movement he has inspired, Mitt Romney, the party’s nominee in 2012, attacked him as “a fraud” and “a phony” who would drive the country to the point of collapse.

“He’s playing the American public for suckers,” Mr. Romney said, breaking from his customary restraint. “He has neither the temperament nor the judgment to be president,” he added.

As soon as he was finished, Senator John McCain, the party’s standard-bearer in 2008, endorsed Mr. Romney’s jeremiad and denounced Mr. Trump as a candidate who was ignorant of foreign policy and has made “dangerous” pronouncements on national security.

For a party that prizes unity and loyalty, it was an unheard-of onslaught against a figure who is marching toward the nomination, highlighting the widening and seemingly unbridgeable gaps between Republican elites and their electorate.

There is a growing prospect the Republican Party leadership could abandon its own nominee this fall, a once unthinkable scenario. Mr. Romney all but explicitly called for a messy convention floor battle, the likes of which neither party has witnessed in decades.

Former Senator Norm Coleman of Minnesota, a supporter of Marco Rubio’s, said that Mr. Trump’s nomination would create a “historic breach” in the Republican Party. “This guy cannot be the president of the United States,” Mr. Coleman said.

Mr. Trump, who claims to relish the disapproval of his party’s elite, embraced his role as a defiant outsider on Thursday. In an immediate and venomous reply, he belittled Mr. Romney’s objections and derided him as a “failed candidate,” “choke artist” and “loser” for his loss to President Obama in 2012.

The mounting hostility between Mr. Trump and traditional party leaders has pushed the party to the edge of rupture. In swift succession, senior Republicans have registered their disapproval by either vowing to withhold support from Mr. Trump in a general election or declining to back him in the primaries. On Thursday, dozens of conservative national security leaders released a letter announcing that they would never vote for Mr. Trump.

Mr. Romney’s diatribe in Salt Lake City encapsulates the crisis. In dire language, he evoked the specter of totalitarianism, saying Mr. Trump embodied a “brand of anger that has led other nations into the abyss.” Such a statement would be impossible to retract in a general election campaign and all but precludes a later endorsement.

But the timing of Mr. Romney’s assault, after Mr. Trump’s commanding electoral victories in seven states on Tuesday, may make it futile. And Mr. Romney’s history with Mr. Trump, which he ignored in his speech on Thursday, could undercut the impact of his warning: Mr. Romney eagerly sought and publicized his endorsement by Mr. Trump in 2012, even as Mr. Trump heckled and harassed Mr. Obama with accusations that he was not born in the United States.

Mr. Trump repeatedly and provocatively reminded his supporters of that endorsement on Thursday, saying Mr. Romney had been prepared to debase himself to obtain it.

“He was begging for my endorsement,” Mr. Trump said. “I could have said, ‘Mitt, drop to your knees’ — he would have dropped to his knees.”

In an apologetic-sounding addendum on Thursday, conveyed over Twitter, Mr. Romney said he would never have accepted Mr. Trump’s blessing in 2012 had Mr. Trump made the same kind of divisive remarks back then.

For Mr. Romney, a confrontation with Mr. Trump was both inevitable and undesirable. The son of a Republican presidential candidate himself, he is a reluctant instigator of ugly intraparty battles. But Mr. Trump represented things that Mr. Romney, a religious family man, loathed: a profane, philandering self-promoter.

Mr. Romney had discussed simply throwing his support behind a mainstream Trump rival, like Mr. Rubio, the Florida senator, to undermine his candidacy, a traditional route for a party elder. But after debating his options with friends and advisers, he decided instead to confront Mr. Trump directly and avoid issuing an endorsement that might saddle a candidate with responsibility for his attacks.

After chiding Mr. Trump last week for declining to release his tax returns, Mr. Romney decided over the weekend to escalate his criticism, several close associates said. Mr. Trump’s hesitation on Sunday in disavowing an endorsement from David Duke, the white supremacist, convinced Mr. Romney that Mr. Trump could never win in November, and that there was no possibility of reaching an accommodation with him in a general election.

And Mr. Romney told friends that he could not stomach “coddling” a bigot.

Mr. Romney tapped out a draft of his speech on his computer in Utah and shared it with his longtime advisers. His disgust suffused the remarks. He upbraided Mr. Trump for boasting about his marital affairs and lacing his speeches with vulgarities. And he questioned Mr. Trump’s basic decency.

“Dishonesty,” he said, “is Trump’s hallmark.”

Even Republicans who welcomed Mr. Romney’s intervention in the race were doubtful that he would be able to dislodge Mr. Trump. “I feel like it’s Trump’s nomination regardless of what anyone says,” said Jason Adam, 37, a technology worker in Detroit. Michigan is the next big state to vote in the presidential race, next Tuesday.

Sam Lopez, 55, a Republican who lives in nearby Shelby Township, saw Mr. Romney’s attack as a case of sour grapes.

“I think what he did was very distasteful,” he said. “To go out and to publicly chastise Trump because Trump is winning: Is that the American way?”

Still, Mr. Romney’s plea may help deter other Republicans from joining forces with Mr. Trump, at a point in the race when party leaders would typically rally around a clear front-runner.

In the kind of defection that worries Mr. Romney, Gov. Chris Christie of New Jersey, once a pillar of the party establishment, bolted to Mr. Trump’s camp last Friday, and three members of Congress have endorsed his candidacy. Despite an outpouring of criticism after his endorsement, Mr. Christie has urged the party to embrace Mr. Trump as its strongest competitor.

Mr. Christie was politely dismissive of Mr. Romney on Thursday and reminded him that democracy did not always conform to the party’s will.

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“It is the people who vote who ultimately decide who the nominee is,” he said. “That’s how Mitt Romney became the nominee.”

On the campus of the University of Utah, in a city where his family has deep roots, Mr. Romney argued that it was still possible to save a party that he and his father helped lead, a generation apart, from becoming the party of Mr. Trump.

He offered a blueprint for a contested convention that may prove impractical, urging Republican primary voters to cast ballots for whichever candidate appeared to be the strongest Trump alternative in each state: Mr. Rubio in Florida, Gov. John Kasich in Ohio and Senator Ted Cruz wherever appropriate.

But time, in fact, is running out for Mr. Trump’s opponents, who must defeat him in several of the big states that vote over the next two weeks to block him from seizing an insurmountable lead.

John F. Lehman, a former Navy secretary who advised Mr. McCain’s 2008 campaign, said the alarm bells about Mr. Trump’s preparedness for the presidency may be ringing too late.

“It’s too bad that the party has waited so long, and the other candidates waited so long, to point out these shortcomings, because they are severe,” Mr. Lehman said. Explaining the delay, Mr. Lehman acknowledged, “People haven’t come out against him because nobody thought he’d get this far.”

Despite the late date, Mr. Romney’s remarks could still sway voters before important primaries in two states that vote on Tuesday: Michigan, where Mr. Romney was raised and his father was governor, and Idaho, where he is a popular figure with fellow Mormons.

Frances F. Townsend, a former Homeland Security adviser to George W. Bush, said she hoped that Thursday would be a turning point in the halting and haphazard struggle to derail Mr. Trump.

“There needed to be a group that would stand up and say, ‘Yes, I am willing to be counted,’ ” said Ms. Townsend, who signed on to the letter rejecting Mr. Trump. “We cannot all be silent, or be Chris Christie and decide, ‘I am going to put myself first and, for political and opportunistic reasons, I am going to endorse this stuff.’ ”

Historians could not recall another time in the last century when the Republican Party’s previous nominees had so harshly attacked a would-be successor. The most recent antecedent, said David Greenberg, an associate professor at Rutgers University, might be the 1912 election, when a former president, Theodore Roosevelt, led an exodus of progressive voters from the Republican Party and ran as a third-party candidate against the incumbent, William Howard Taft.

“There probably hasn’t been this level of personal invective by one Republican nominee against another leading candidate,” Mr. Greenberg said. “Ever.”

Alan Rappeport and Jonathan Martin contributed reporting from Washington, and Alex Thompson from Salt Lake City.

Transcript of Mitt Romney’s Speech on Donald Trump The following is a transcript of Mitt Romney’s remarks addressing the candidacy of Donald J. Trump, as transcribed by the Federal News Service.