Former presidential candidate Mitt Romney will vote for Texas Senator Ted Cruz, saying he is “repulsed” by Republican front-runner Donald Trump.
Mr Romney said in a Facebook post that the only way to nominate a Republican is to have an open convention, in which party officials choose the nominee.
He campaigned with Governor John Kasich in Ohio but said voting for Mr Cruz is the only way to stop “Trumpism”.
He joins other Republican leaders coalescing around Mr Cruz.
Mr Trump has won the most state contests and holds 678 delegates – 1,237 are needed to win the nomination.
“Mitt Romney is a mixed up man who doesn’t have a clue. No wonder he lost!” Mr Trump said on Twitter.
However Republican leaders are concerned that his controversial comments about immigrants, women and Muslims would make him a weak candidate in the general election in November.
Some also feel that the onetime Democrat cannot be trusted to implement conservative policies.
“Today, there is a contest between Trumpism and Republicanism,” Mr Romney said. “Through the calculated statements of its leader, Trumpism has become associated with racism, misogyny, bigotry, xenophobia, vulgarity and, most recently, threats and violence. I am repulsed by each and every one of these.”
Analysis: Anthony Zurcher, BBC North America Reporter
It turns out Mitt Romney’s speech roundly condemning Donald Trump two weeks ago was just an opening salvo in what could be a long war against the New York businessman.
With the Utah primary days away, the 2012 Republican nominee, still highly respected among the state’s large Mormon population, is casting his lot with – and personal ballot for – Ted Cruz in a last-ditch attempt to stop “Trumpism” from taking over his party.
Mr Romney and the Texas senator are certainly strange political bedfellows. One is the face of the party’s genteel establishment; the other a bomb-throwing backbencher who has spent more time condemning his party’s leadership than courting it.
The goal for Mr Romney continues to be a political street fight at an open Republican convention. The former Massachusetts governor has apparently concluded that this necessitated sticking the knife in Mr Trump’s other opponent, Ohio Governor John Kasich, and going all-in with Mr Cruz.
His efforts prior to the last round of voting did little to slow Mr Trump’s momentum. It remains to be seen if his latest moves will be any different.
Earlier, he gave a speech outlining why he was against Donald Trump, a billionaire businessman with no political experience, calling him a “phoney” and a “fraud”.
Mr Romney’s home state of Utah holds its presidential contest on Tuesday.
A group of conservatives including well-known talk radio host Erick Erickson met on Thursday to discuss ways to defeat Donald Trump, including launching a third party campaign to challenge the New York businessman.
“We encourage all former Republican candidates not currently supporting Trump to unite against him and encourage all candidates to hold their delegates on the first ballot,” he said in a statement, put out on behalf of the group.
“We believe that the issue of Donald Trump is greater than an issue of party. It is an issue of morals and character that all Americans, not just those of us in the conservative movement, must confront.”
More on the US presidential race
Where did Marco Rubio go wrong? – Tuesday was a bruising night for the establishment hope, and other takeaways from our US correspondents
Jorge Ramos on Donald Trump’s rise – Meet the Republican front-runner’s nemesis
Who is funding the US election? – Money is a big issue in the 2016 US presidential race
Many party members have also misgiving about Mr Cruz because he has repeatedly and publicly denounced Republican leaders.
However, more prominent Republicans are throwing their support behind Mr Cruz in a last-ditch effort to stop Mr Trump.
Popular North Carolina Governor Nikki Haley and senators Mike Lee of Utah Lindsey Graham of South Carolina have recently endorsed Mr Cruz.
Florida Senator Marco Rubio, who left the Republican race on Tuesday, said this week would not be endorsing any of his former rivals.
He also said he had no interest in becoming a vice-presidential nominee.
Mr Trump won four out five primaries on Tuesday, but the race in Missouri has not been called for the Republicans yet.
Democrat Hillary Clinton narrowly won the state’s Democratic primary after her opponent Vermont Senator Bernie Sanders decline to pursue a recount.