By Steve Holland
DETROIT (Reuters) - Former U.S. presidential candidate Mitt Romney attacked 2016 Republican front-runner Donald Trump as “a fraud” on Thursday and called for tactical voting in primaries to stop him as the party slid closer to civil war over the outspoken New York billionaire’s White House run.
In an unusually harsh speech, party elder Romney warned that former reality TV star Trump would likely lose to possible Democratic candidate Hillary Clinton in the Nov. 8 election if he becomes the Republican nominee.
Trump's rise has split the Republican Party between mainstream figures like Romney, and Trump supporters who complain the party does not reflect their concerns about illegal immigration, the slow economic recovery and what they see as America's diminishing role in the world.
That split widened when Romney, the party nominee in 2012, urged Republican primary voters to vote tactically in different states to back Trump's opponents and block his path to the nomination.
"Here's what I know. Donald Trump is a phony, a fraud," said Romney, 68.
"I would vote for Marco Rubio in Florida, for John Kasich in Ohio, and for Ted Cruz or whichever one of the other two contenders has the best chance of beating Mr. Trump in a given state," he said.
Trump, 69 has made his party's establishment uneasy with his abrasive tone and policy positions, including plans to build a wall on the U.S.-Mexican border, deport 11 million illegal immigrants and temporarily bar Muslims from entering the country.
Romney's speech in Utah was the spearhead of a mainstream Republican attempt to rein in Trump after he won most states in this week's Republican Super Tuesday nominating contests and took a step toward earning the nomination.
Trump also leads many polls for primaries in the remaining states, including in major ones like Florida on March 15, dampening prospects of derailing him.
The party establishment's strategy risks backfiring by further energizing Trump's supporters, many of them white, blue-collar voters. Romney did not endorse any candidate.
Conservative political commentator Michelle Malkin said she was unimpressed with Romney. "If only Romney talked like this four years ago about Obama ... or Trump," Malkin tweeted. "Too freaking late and too freaking lame."
Trump dismissed the former Massachusetts governor who lost to Democratic President Barack Obama four years ago. "Mitt is a failed candidate. He failed. He failed horribly. He failed badly," Trump told a rally in Maine.
Romney's speech lit up social media, with "Mitt Romney" becoming one of the top-trending topics on Twitter in the United States. Twitter users posted about Romney roughly 38 times a second following the speech, according to Zoomph, the social
media analytics firm.
Romney decided on his own to give the speech - which he wrote himself - and was not asked to do so by the Republican National Committee, an RNC official said.
Romney said Trump's economic policy would sink America "into prolonged recession," mocked Trump's ego, and called him a "con man."
"A business genius he is not," Romney said.
David Axelrod, a former senior adviser to Obama, called the Romney speech a "break glass" moment he had not seen since 1964, when Republicans abandoned their candidate Barry Goldwater.
Axelrod noted thousands of Republicans had already voted for Trump in primary elections. "I wonder about tactic of calling them fools," Axelrod wrote on Twitter.
Earlier, more than 70 Republican national security leaders signed a scathing open letter opposing Trump and his stance on many foreign policy issues.
Romney also pointed to Trump's refusal to release his tax returns and initial reluctance to disavow an endorsement from a former leader of the Ku Klux Klan white supremacist group.
Romney's speech came hours before Trump and his rivals share a stage in Detroit at 9 p.m. EST for a debate hosted by Fox News.
Trump, who has self-funded his primary campaign, would turn to donors in the general election if he wins the party's nomination, CNN reported.
Also on Thursday, Florida Governor Rick Scott said he would not endorse anyone in the Republican presidential race, despite the candidacy of Rubio, a U.S. senator from that state.
(Additional reporting by Susan Heavey, Megan Cassella, Eric Beech, James Oliphant; Writing by Steve Holland, Roberta Rampton and Alistair Bell; Editing by Lisa Von Ahn and Peter Cooney)