DETROIT (Reuters) - Former U.S. presidential candidate Mitt Romney attacked 2016 Republican front-runner Donald Trump as “a fraud” on Thursday and urged primary voters to keep the outspoken New York billionaire from getting the nomination, paving the way for possible horse trading at a party convention in July. 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, who did not endorse any candidate.  “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. Rubio is a U.S. senator from Florida and Kasich is the Ohio governor. By calling for targeted voting, Romney was setting up the possibility of a contested convention when Republicans gather in Cleveland in mid-July to select their nominee for the November election to succeed Democratic President Barack Obama. That could create a pathway to deny Trump the 1,237 delegates needed for nomination. The last Republican convention to go beyond one ballot was  in 1948 when Thomas Dewey was nominated. “I think the governor is just being realistic about where things stand and advocating a potential strategy that could stop the Trump nomination,” said former Romney spokesman Ryan Williams. Republican strategist Scott Reed said he doubted the last-ditch tactical voting suggestion would work. “No one will be playing the targeted voting game. There’s too much anger and distrust,” Reed 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. More than 90 Republican national security leaders have signed a scathing open letter opposing Trump and his stance on many foreign policy issues.     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.  The address came hours before Trump and his rivals shared a stage in Detroit at 9 p.m. EST for a debate hosted by Fox News.  Trump 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. “If only Romney talked like this four years ago about Obama ... or Trump,” conservative political commentator Michelle Malkin said on Twitter. “Too freaking late and too freaking lame.” Trump dismissed the former Massachusetts governor who lost to Obama four years ago. “Mitt is a failed candidate. He failed. He failed horribly. He failed badly,” Trump told a rally in Maine.        Romney decided on his own to give the speech, which he wrote himself. 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.  (Additional reporting by Doina Chiacu,   Eric Beech, Warren Strobel, Jonathan Landay and Matt Spetalnick in Washington; Writing by Steve Holland, Roberta Rampton; Editing by Alistair Bell and Peter Cooney) This article was funded in part by SAP. It was independently created by the Reuters editorial staff. SAP had no editorial involvement in its creation or production.