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 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," 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, Sen. John McCain, the party's standard-bearer in 2008, endorsed Romney's jeremiad and denounced Trump as a candidate who was ignorant of foreign policy and has made "dangerous" pronouncements on national security.
For a party concerned with 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 leaders and their electorate.
There is a growing prospect the Republican Party leadership could abandon its own nominee this fall, a once unthinkable scenario. Romney all but explicitly called for a messy convention floor battle, the likes of which neither party has witnessed in decades.
Former Sen. Norm Coleman of Minnesota, a supporter of Marco Rubio's, said that Trump's nomination would create a "historic breach" in the Republican Party. "This guy cannot be the president of the United States," Coleman said.
Trump, who claims to relish the disapproval of his party's elite, embraced his role as a defiant outsider on Thursday. In an immediate, venomous reply, he belittled Romney's objections and derided him as a "failed candidate," "choke artist" and "loser" for his loss to President Barack Obama in 2012.
The mounting hostility between Trump and traditional party leaders has pushed the party to the edge of complete rupture. In swift succession, senior Republicans have registered their disapproval by either vowing to withhold support from 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 Trump.
Romney's diatribe in Salt Lake City encapsulates the crisis. In his direst language, he evoked the specter of totalitarianism, saying 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 Romney's assault, after Trump's commanding electoral victories in seven states on Tuesday, may make it futile. And Romney's history with Trump, which he ignored in his speech on Thursday, could undercut the effect of his warning: Romney eagerly sought and publicized his endorsement by Trump in 2012, even as Trump heckled and harassed Obama with accusations that he was not born in the United States.
Trump repeatedly and provocatively reminded his supporters of that endorsement on Thursday, saying Romney had been prepared to debase himself to obtain it.
"He was begging for my endorsement," 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, Romney said he would never have accepted Trump's blessing in 2012 had Trump made the same kind of divisive remarks back then.
For Romney, a confrontation with Trump was both inevitable and undesirable. The son of a Republican presidential candidate himself, he is a reluctant instigator of ugly intraparty battles. But friends said that Trump represented everything that Romney, a devoted religious family man, loathed: a profane, philandering self-promoter.
Romney had discussed simply throwing his support behind a mainstream Trump rival, like 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 Trump directly and avoid issuing an endorsement that might saddle a candidate with responsibility for his attacks.
After chiding Trump last week for declining to release his tax returns, Romney decided over the weekend to escalate his criticism, several of his close associates said. Trump's hesitation on Sunday in disavowing an endorsement from David Duke, the white supremacist, convinced Romney that Trump could never win in November, and that there was no possibility of reaching an accommodation with him in a general election.
And Romney told friends that he could not stomach "coddling" a bigot.
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 Trump for two profoundly un-Romney-like habits: boasting about his marital affairs and for lacing his speeches with vulgarities. And he questioned Trump's basic decency.
"Dishonesty," he said, "is Trump's hallmark."
Even Republicans who welcomed Romney's intervention in the race were doubtful that he would be able to dislodge 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 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, Romney's plea may help deter other Republicans from joining forces with 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 Romney, Gov. Chris Christie of New Jersey, once a pillar of the party establishment, bolted to Trump's camp last Friday, and three members of Congress have endorsed his candidacy. Despite an outpouring of criticism after his endorsement, Christie has urged the party to embrace Trump as its strongest competitor.
Christie was politely dismissive of Romney on Thursday and reminded him that democracy did not always conform to the party's will.
"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, Romney argued, again and again, that it was still possible to save a party that he and his father helped lead, a generation apart, from becoming the party of 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: Rubio in Florida, Gov. John Kasich in Ohio and Sen. Ted Cruz wherever appropriate.
But time, in fact, is running out for 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 McCain's 2008 campaign, said the alarm bells about 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," Lehman said.
Explaining the delay, Lehman acknowledged, "People haven't come out against him because nobody thought he'd get this far."
Despite the late date, Romney's remarks could still sway voters before important primaries in two states that vote on Tuesday: Michigan, where 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 Trump.
"There needed to be a group that would stand up and say, 'Yes, I am willing to be counted,'" said Townsend, who signed on to the letter rejecting 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, a historian 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 president, William Howard Taft.
"There probably hasn't been this level of personal invective by one Republican nominee against another leading candidate," Greenberg said. "Ever."