Romney's speech comes as the Republican Party grapples with the distinct possibility that Trump will become the party's nominee. While some Republicans have come around to Trump's candidacy, a forceful Republican effort to take down Trump has failed to materialize.
Romney last-ditch bid to save his party from Trump started with tweets knocking Trump's comments on the Ku Klux Klan and calling on him to release his tax returns.
His slashing speech Thursday was remarkable not simply for the unparalled specter of the party's last nominee savaging its frontrunner but for the dire picture Romney painted of a Trump presidency and what it would do to the country.
Romney said that if Trump's economic plans "were ever implemented, the country would sink into prolonged recession."
"But you say, 'wait, wait, wait. Isn't he a huge business success? Doesn't he know what he's talking about?' No, he isn't. And no, he doesn't," he continued, before listing failed Trump businesses like Trump Vodka and Trump Airlines.
The former nominee urged Republicans to nominate Sen. Ted Cruz (R-TX), Sen. Marco Rubio (R-FL), or Ohio Gov. John Kasich.
"If the other candidates can find some common ground, I believe we can nominate a person who can win the general election and who will represent the values and policies of conservatism," Romney said.
Romney warned that if Republicans nominate Trump, Hillary Clinton will become president.
"For the last three decades the Clintons have lived at the intersection of money and politics, trading their political influence to enrich their personal finances. They embody the term crony capitalism. It disgusts the American people, and causes them to lose faith in our political process," he said. "A person who untrustworthy and dishonest as Hillary Clinton must not become president."
Romney encouraged Republicans to enact a voting strategy to deny Trump major primary wins.
"Given the current delegate selection process, that means that I'd vote for Marco Rubio in Florida and for John Kasich in Ohio, and for Ted Cruz or whoever has the best chance to beating Mr. Trump in a given state," Romney said.
This would presumably keep Trump from winning a majority of delegates and lead to a contested convention in July.
The former Republican nominee pointed to specific comments made by Trump throughout the campaign that he thinks reflect poorly on the real estate mogul's character.
He bashed Trump’s July comments that dismissed Sen. John McCain (R-AZ) as a "war hero because he was captured." Romney said that there is "dark irony in his boasts of his sexual exploits during the Vietnam war" while McCain was imprisoned and tortured.
He also listed what he said were Trump's dishonest moments, painting Trump as an unstable person.
"Dishonesty is Donald Trump's hallmark. He claimed that he had spoken clearly and boldly against going into Iraq. Wrong. He spoke in favor of invading Iraq. He said he saw thousands of Muslims in New Jersey celebrating 9/11. Wrong. He saw no such thing. He imagined it," Romney said. "He's not of the temperament of the kind of stable, thoughtful person we need as leader. His imagination must not be married to real power."
He described Trump as "a con man, a fake," referencing Trump's interview avoiding disavowal of the KKK and a New York Times interview that allegedly shows Trump's stance on immigration is flexible.
"Mr. Trump has changed his positions, not just over the years, but over the course of the campaign, and on the Ku Klux Klan, daily for three days in a row. We will only know if he's the real deal or a phony if he releases his tax returns and his tape of the interview with the New York Times. I predict there are more bomb shells in his tax returns," Romney said. "I predict he told The New York Times that his immigration talk is just that, talk."
Romney asked Republicans to “think of Donald Trump's personal qualities — the bullying, the greed, the showing off, the misogyny, the third grade theatrics."
He then mocked Trump’s nickname “The Donald.”
“He's the only person in the entire country who whom we have added an article before his name, and it wasn't because he had attributes we admired,” Romney said. “Now, imagine your children and your grandchildren acting the way he does. Would you welcome that? Haven't we seen before what happens when people in prominent positions fail the basic responsibility of honorable conduct? We have. And it always injures our families and our country.”
The former Republican nominee predicted that Trump would respond to his speech with insults.
"Will he talk about our policy differences? Or will he talk me with every imaginable low road insult?" Romney asked. "This may tell you what you need to know about his temperament, his stability, and his suitability to be president."
Romney concluded his speech by calling on Americans not to fall for Trump's rhetoric.
"I understand the anger Americans feel today. In the past, our presidents have channelled that anger and forged it into resolve," he said. "Our anger was transformed into energy directed for good. Mr. Trump is directing our anger for less than noble purposes."
"This is the very brand of anger that has led other nations into the abyss," Romney added.
"Here's what I know. Donald Trump is a phony. A fraud. His promises are as worthless as a degree from Trump University," Romney continued. "He's playing the American public for suckers. He gets a free ride to the White House, and all we get is recession. His foreign policies would make America and the world less safe. He has neither the temperament nor the judgment to be president. And his personal qualities would mean that America would cease to be a shining city on a hill."