Former Republican U.S. presidential nominee Mitt Romney speaks critically about current Republican presidential candidate Donald Trump during speech in Salt Lake City.

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.

File photo of businessman and real estate developer Donald Trump greeting U.S. Republican presidential candidate and former Massachusetts Governor Romney after endorsing his candidacy for president at the Trump Hotel in Las Vegas.

Related articles across the web

• Establishment Republicans start to realize it may be too late to stop Trump
• 2016 Debate Ranker So Far: GOP Averaging 15.8 Million Viewers
• John Kasich visits Atlanta on Tuesday, facing an uphill battle in Georgia
• A Shrinking GOP Primary Field Might Not Be Enough to Stop Trump - Here's the Proof
• Spoiler: Jeb Bush Does Not Win Nevada
• Donald Trump wins by ten points in South Carolina
• Arizona poll: Trump dominates, while Rubio surges above Cruz
• Nikki Haley Is Trying to Help Marco Rubio Get His Mojo Back
• It's The Anger, Stupid: Gingrich Sizes Up South Carolina
• Carly Fiorina Doesn't Get Invite to Next GOP Debate