This story has been updated.

SALT LAKE CITY — Mitt Romney, the 2012 Republican presidential nominee, delivered a sweeping point-by-point indictment of Donald Trump on Thursday and implored Republicans to reject the businessman’s candidacy in an election "that will have profound consequences for the Republican Party and, more importantly, for the country."

"Donald Trump is a phony, a fraud," Romney told nearly 700 people at the University of Utah. "His promises are as worthless as a degree from Trump University. He's playing members of the American public for suckers: He gets a free ride to the White House, and all we get is a lousy hat."

Trump fired back a few hours later at a rally in Portland, Maine, bemoaning Romney's "nasty" remarks and dismissing him as a "choke artist" and "failed candidate" who was eager for Trump's endorsement in 2012.

"You can see how loyal he is," Trump said. "He was begging for my endorsement. I could’ve said, 'Mitt, drop to your knees,' and he would’ve dropped to his knees. He was begging. True. True. He was begging me."

There was little precedent for Romney’s remarks: Never before in modern political history has the immediate past nominee of a party delivered an entire speech condemning the current front-runner. Sen. John McCain (R-Ariz.) – the party’s 2008 nominee – later said he shared Romney’s concerns.

Most boldly, Romney called for a scenario that likely would lead to a convention floor flight, recommending that voters cast ballots for Sen. Marco Rubio in his home state of Florida, for Gov. John Kasich in Ohio, and everywhere else, for the candidate best positioned to deny Trump a win.

From issues domestic and foreign to those of moral character and temperament, Romney called Trump ill-qualified to serve as president. His remarks to the university’s Hinckley Institute of Politics called out Trump for his many failed businesses, including airlines, vodka and a mortgage company. He raised concerns about Trump's sexual exploits, his three marriages and his taunts toward the disabled, Mexican immigrants and female journalists and politicians.

“But you say, ‘Wait, wait, wait, isn’t he a huge business success? Doesn’t he know what he’s talking about?’ ” Romney said. “No, he isn’t. And no, he doesn’t.”

The 18-minute address was unlike anything other GOP presidential candidates have delivered. It served as a public airing of concerns Romney has shared in brief spurts on social media or privately to friends and supporters. The speech came at a critical juncture for the Republican Party, with Trump's seemingly unstoppable march to the nomination setting off panic this week among party leaders fearful that the New Yorker’s ascendance could cost them the general election, spoil the chances for down-ballot candidates and irreparably tarnish the party’s brand.

The auditorium was packed mostly with college students, but two of Romney’s grown sons, Josh and Ben, were in the audience, as was Utah Gov. Gary Herbert, Lt. Gov. Spencer J. Cox and their spouses. At least four Trump supporters were also in the crowd and taunted Romney from their seats.

“Donald Trump tells us that he is very, very smart,” Romney said at one point.

“Smarter than you!” someone shouted from the back of the auditorium.

When Romney concluded, a man wearing a red “Make America Great Again” hat stood to leave, waving a Trump campaign sign. Another shouted at Romney as he shook hands with fans in the front row.

As results showed Donald Trump leading in at least six states on Super Tuesday, Sens. Ted Cruz (R-Tex.) and Marco Rubio (R-Fla.) argued that nominating him would be bad for the Republican party. Here are key moments from their speeches following the March 1 races. (Sarah Parnass/The Washington Post)

Romney briefly considered running again before announcing early last year that he would take a pass. His speech in Utah, where he has a home, neither launched a fresh campaign nor served as an endorsement of any candidate.

"Instead, I’d like to offer my perspective on the nominating process in my party," he said.

Dozens of national media outlets descended on the college campus to hear from the former presidential contender. The speech garnered national television coverage, with cable news channels devoting hours to the governor’s remarks. Even local television stations preempted regular programming to broadcast the words of a famous resident who led the city’s 2002 Olympic organization.

"In 1964, days before the presidential election, Ronald Reagan went on national television and challenged America that it was a 'Time for Choosing,' " Romney said. "He saw two paths for America, one that embraced conservative principles dedicated to lifting people out of poverty and helping create opportunity for all, and the other, an oppressive government that would lead America down a darker, less-free path.

"I'm no Ronald Reagan and this is a different moment, but I believe with my heart and soul that we face another time for choosing,” he said.

He faulted Trump for offering “very few specific economic plans” and said that “what little he has said is enough to know that he would be very bad for American workers and for American families.”

On national security, Romney said that Trump’s “bombast is already alarming our allies and fueling the enmity of our enemies.”

“The only serious policy proposals that deal with a broad range of national challenges that we confront today come from Ted Cruz, Marco Rubio and John Kasich,” Romney said, referencing the other remaining GOP candidates.

“I know that some people want this race to be over,” he added. “They look at history and say a trend like Mr. Trump’s isn’t going to be stopped. Perhaps. But the rules of political history have pretty much all been shredded during this campaign.”

Romney urged Cruz, Kasich and Rubio to “find some common ground” and help the party “nominate a person who could win the general election and who will represent the values and policies of conservatism.”

Throughout the speech Romney repeatedly dismissed Trump’s personal behavior: “Think of Donald Trump’s personal qualities. The bullying. The greed. The showing off. The misogyny. The absurd third-grade theatrics.”

“Watch, by the way, how he responds to my speech today,” he added. “Will he talk about our policy differences? Or will he attack me with every imaginable low-blow insult?”

True to form, Trump mocked Romney's failed 2012 bid against President Obama and called him a “choke artist.” Trump also spent several minutes telling supporters that he is wealthier than Romney.

“He is a failed candidate. He failed horribly. That was a race, I have to say folks, that should have been won,” Trump said.

"With Mitt, I just wanted to tell you that he came out and he was very nasty. I thought he was a better person than that," Trump added later.

Romney never mentioned in his speech that he had eagerly sought and received Trump’s endorsement in 2012. Ultimately, Romney traveled to Trump’s lavish Las Vegas hotel to hold a news conference and accept his support.

“Being in Donald Trump's magnificent hotel and having his endorsement is a delight,” Romney said the day of the endorsement.

Aides said at the time that Romney embraced Trump, however reluctantly, to keep him in the fold and from bolting the party. Trump emerged as a vocal Romney surrogate, recording more than 30 “robo-calls” and doing more than 50 radio interviews in states that held early primaries. Trump and his wife, Melania, also hosted a lavish 63rd birthday party for Ann Romney that doubled as a campaign fundraiser. More than 400 supporters paid at least $1,000 to attend the event at Trump Tower in New York.

But on Thursday, Romney left no doubt about Trump: “He has neither the temperament nor the judgment to be president. And his personal qualities mean that America would cease to be a shining city on a hill.”

Afterward, McCain agreed.

“I want Republican voters to pay close attention to what our party's most respected and knowledgeable leaders and national security experts are saying about Mr. Trump, and to think long and hard about who they want to be our next Commander-in-Chief and leader of the free world,” McCain said in a statement.

But House Speaker Paul D. Ryan (R-Wis.), Romney’s 2012 running mate, declined to back Romney’s remarks, telling Fox News, "I'm gonna keep my powder dry" to respect his role as chairman of the Republican National Convention.

"People who are in the party are going to be speaking their minds while we are selecting a nominee, and so everything is fair game on the way to the nomination," he said.

Jose DelReal in Portland, Maine; Philip Rucker in Detroit; and Sarah Larimer in Washington contributed to this report.