"Here’s what I know. Donald Trump is a phony, a fraud."
Mitt Romney, as expected, delivered a scathing speech against Donald Trump, issuing a Republican call to arms and asking voters to "make the right choice" and vote for anyone but the brash businessman.
The 2012 Republican presidential nominee neither announced his candidacy nor endorsed any of the three remaining Republicans, but warned of a dark future if Mr. Trump became the party's presidential nominee.
"Here's what I know. Donald Trump is a phony, a fraud," Mr Romney said. "His promises are as worthless as a degree from Trump University. He's playing the American public for suckers: He gets a free ride to the White House and all we get is a lousy hat."
"America is poised to lead the world for another century," Mr Romney said, but added that this is only if Mr Trump isn't representing the Republican party. He then said any of Ted Cruz, Marco Rubio or John Kasich should be the Republican nominee.
"A business genius he is not," Mr Romney said before naming a number of Mr Trump's failed business endeavors. "He is not of the temperment" to be president.
Prominent Republicans have applauded the speech and are stepping up to denouce Mr Trump.
"Donald Trump tells us that he is very, very smart. I'm afraid that when it comes to foreign policy he is very, very not smart," Mr Romney said.
"Think of Donald Trump's personal qualities, the bullying, the greed, the showing off, the misogyny, the absurd third grade theatrics. We have long referred to him as "The Donald." He is the only person in America to whom we have added an article before his name. It wasn't because he had attributes we admired."
The former governor of Massachusetts also took a shot at Democratic front-runner Hillary Clinton, calling her and husband Bill Clinton the picture of crony capitalism, but focused most of his attacks on Mr Trump.