Mitt Romney, the Republican presidential nominee four years ago, attempted to eviscerate Donald Trump on Thursday in an 18-minute speech that questioned the GOP front-runner's temperament, intelligence and honesty.

It was a sober, direct attack — tougher than Trump has faced from any other Republican in this campaign.

But tucked in Romney's takedown was a shaky path forward. He didn't endorse any of the other three Republicans in the race — Florida Sen. Marco Rubio, Texas Sen. Ted Cruz or Ohio Gov. John Kasich. Romney instead said he would vote for anyone who could beat Trump in any state, essentially a route that he hopes would lead to a rare brokered convention, where Republicans could try to wrestle away the nomination from Trump. He also hoped the remaining candidates can coordinate into an anyone-but-Trump brigade.

"If the other candidates can find common ground, I believe we can nominate a person who can win the general election and who will represent the values and policies of conservatives," Romney said at the University of Utah's Gardner Hall before a crowd of 680, mostly students, including some standing in the stairways. Romney's sons Josh and Ben were in attendance, as was Gov. Gary Herbert, Lt. Gov. Spencer Cox and U. President David Pershing.

Romney, who briefly flirted last year with a third presidential bid, didn't hint that he would seek the GOP nomination. He instead said one of the other three candidates should become the party's standard-bearer.

Romney's team asked the U.'s Hinckley Institute of Politics to host a forum at which he could take on Trump, scheduling it the same day as the Fox News GOP debate in Detroit. It comes just days before big states such as Florida and Ohio hold their primary elections, offering one of the last chances for the party to change course.

The speech drew national reporters off of the campaign trail and attracted international attention, a nod to how unprecedented it was for a former nominee to attempt to take out a surging candidate.

In this case, Romney sought and received Trump's endorsement in 2012. Romney didn't mention that in his speech, though Trump brought it up as he sought to defend himself during remarks in Maine in which he called the 2012 GOP nominee a "failed candidate" and a "choke artist."

"He was begging for my endorsement" Trump said. "I could have said, 'Mitt, drop to your knees.' He would have dropped to his knees."

Romney responded on Twitter: "If Trump had said 4 years ago the things he says today about the KKK, Muslims, Mexicans, disabled, I would NOT have accepted his endorsement."

In his speech, Romney didn't just go after Trump, he also criticized Hillary Clinton, saying the Democratic front-runner has traded her political influence for personal gain and criticizing her service as secretary of state. But even these assessments were used as tools to argue against a Trump nomination.

"A person so untrustworthy and dishonest as Hillary Clinton must not become president," he said. "But a Trump nomination enables her victory."

Romney's critique of Trump was broad and direct, veering into the name-calling that has come to dominate the raucous GOP primary.

"Here's what I know. Donald Trump is a phony, a fraud," Romney said to cheers. "He's playing the American public for suckers. He gets a free ride to the White House, and all we get is a lousy hat."

Romney didn't stop there, he gave specifics.

He said Trump's tax plan would balloon the deficit and lead to a recession. He said his national security statements would hurt relations with allies and fuel America's enemies.

"He would be very bad for American workers and American families," Romney said. "And when it comes to foreign policy, he is very, very not smart."

Romney said Trump uttered "the most ridiculous and dangerous idea of the campaign season," when, on "60 Minutes," he said that the United States should let the Islamic State terrorist group take control of Syria.

"This is recklessness in the extreme," he said.

Romney, himself a multimillionaire, questioned Trump's business success, noting the failures of Trump Airlines, Trump University and Trump Mortgage.

"A business genius he is not," Romney said.

He questioned Trump's temperament, reminding the crowd that the GOP front-runner mocked a disabled reporter, attributed Fox News' Megyn Kelly's tough questions to her menstrual cycle and bragged about his marital affairs.

Romney questioned Trump's sincerity, suggesting that the candidate has changed positions — or at least hinted that he's willing to — on issues such as immigration and releasing his tax returns.

Romney said he understood the anger felt by American voters and that presidents have previously channeled such anger into purpose.

"Mr. Trump is directing our anger for less-than-noble purposes. He creates scapegoats of Muslims and Mexican immigrants. He calls for the use of torture and for killing the innocent children and family members of terrorists. He cheers assaults on protesters. He applauds the prospect of twisting the Constitution to limit First Amendment freedom of the press," he said. "This is the very brand of anger that has led other nations into the abyss."

There was only one step Romney didn't take and that would be to say he wouldn't vote for Trump if he was the nominee. Romney confidant and former Hinckley Institute Director Kirk Jowers said there was good reason for that. Romney doesn't want to see Clinton become president and he knows that the ideological tilt of the U.S. Supreme Court is likely at stake in November.

Sen. John McCain, R-Ariz, the Republican nominee in 2008, released a statement after Romney's speech saying he held the same concerns.

"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," McCain said, "and to think long and hard about who they want to be our next commander in chief and leader of the free world."

While Romney received a mostly enthusiastic response, at least two men in the back of the auditorium heckled him, urging him to "go home." The crowd quickly came to the speaker's defense with chants of "Mitt."

The big question will be if Romney's remarks or any move by members of the Republican establishment will blunt Trump's momentum. Hinckley Institute Director Jason Perry said polls show Trump voters "are locked," but he noted there remains a slim chance to stop the front-runner.

Romney's speech follows some Twitter sniping with Trump in which he suggested there was a "bombshell" in Trump's tax returns. Perry said those clashes led Romney to try a more direct assault and helped to draw attention to the speech.

The first people in line for the 9:30 a.m. U. address arrived at Gardner Hall at 6:30 a.m.

Eliza Carr saw this as an opportunity to witness the political theater she sees on television. She likes Romney well enough, but strongly opposes Trump.

Part of it is his style — she said he is "super-obnoxious" — and part of it is a concern for how he could hinder the country if elected.

"He could get us into a lot of trouble."

Austin Fulton, a Republican who is also studying political science, said his concern is that Trump doesn't help the GOP expand its reach.

mcanham@sltrib.com

Reporters Thomas Burr and Christopher Smart contributed to this story.