Mitt Romney attacks donald trump 2016
Former Republican U.S. presidential nominee Mitt Romney criticizes Donald Trump during a speech in Salt Lake City, Utah REUTERS/Jim Urquhart

Former Republican party presidential nominee Mitt Romney has launched another ferocious attack on 2016 frontrunner Donald Trump, calling him "a phony, a fraud" who would make America less safe if he became president. Republicans should choose one of the other candidates to take on the Democratic Party or run the risk of Hillary Clinton entering the White House, he said.

"The only serious policy proposals that deal with the broad range of national challenges we confront today, come from Ted Cruz, Marco Rubio, and John Kasich. One of these men should be our nominee," Romney said in a speech at the University of Utah. He added that Trump did not have "the temperament of a stable, thoughtful leader. His imagination must not be married to real power."

Romney made similar comments last week and so far Trump has made little response to his latest attack, saying only on Twitter: "Teaching the GOP [Grand Old Party] how to lose four years ago was not enough. Now Romney wants to do it again."

The prospect of Donald Trump becoming the GOP nominee is now increasingly likely following his strong showing on Super Tuesday, which left Cruz, Rubio and Kasich floundering and Ben Carson withdrawing from the race. Some Republicans are so concerned about the renegade billionaire they are openly talking about not voting - or, almost unthinkably, voting for Hillary Clinton.

However Trump does still have his supporters among senior Republicans. In a press briefing in Trenton, New Jersey Governor Chris Christie has defended endorsing Trump as nominee, saying he genuinely believes he is the best person to beat Hilary Clinton. Christie also addressed the many comments about his apparent discomfort as he stood behind Trump onstage recently.

"No, I wasn't being held hostage. No, I wasn't sitting up there thinking, 'Oh my God, what have I done?'" said Christie. "I was sitting up there — standing up there — supporting the person who I believe is the best person to beat Hillary Clinton, of the remaining Republican candidates, and it's why I endorsed him. I understand everybody had a lot of fun with it. It doesn't matter to me."