Mitt Romney, the 2012 Republican candidate for US president, on Thursday called current Republican frontrunner Donald Trump a "phony and a fraud" in a speech at the University of Utah.
A Trump candidacy would have "profound consequences" for the nation and the world, Romney said.
"The prospect for a safe and prosperous future are greatly diminished" by a Trump presidency, the former Massachusetts governor said.
The speech comes as the Republican party leadership is pulling out all the stops to halt Trump's candidacy in the wake of his success on Super Tuesday.
Romney alluded to the 1964 election, in which Republican candidate Barry Goldwater, an extreme conservative, was soundly defeated by Democrat Lyndon Baines Johnson.
Romney cast doubt on Trump's business abilities - considered one his strong suits - saying the real estate mogul had inherited his business then suffered a number of bankruptcies such as Trump airlines, Trump vodka and Trump University.
"His promises are as worthless as a degree from Trump University," Romney said, a reference to assertions that Trump defrauded up to 5,000 students at the university that carried his name.
"He's playing the American people for suckers," Romney said, calling Trump a "con-man."
Romney said Trump's domestic policies would lead to a recession, and his foreign policies would make the US and the world less secure.
He said Trump's "bombast" on foreign policy matters was "ridiculous and dangerous" and "already alarming our allies."
He criticized Trump's professed admiration for Russian President Vladimir Putin and his denunciation of George W Bush as a liar, calling it "a twisted example of evil trumping good."
"He has neither the temperament nor the judgement to be president," Romney said, pointing to Trump's bullying, expressions of misogyny and "absurd third-grade theatrics."
Romney offered a veiled warning that Trump's support of torture and tendency to scapegoat minorities had "led other nations into the abyss."