"I don't think he has the temperament to be president and so I want to see one of the other three [Republican candidates] become the nominee".

The debate quickly devolved into the shouting over each other that has marked many pointed exchanges. "He's like 6'2, which is why I don't understand why his hands are the size of someone who is 5'2", Rubio said in Virginia on Sunday. A recent article in New York Magazine reported that Fox News has cooled on Rubio and some within the channel are coming around to Trump.

US Republican presidential candidate Donald Trump's bombast has been a hallmark of the 2016 campaign, but he stooped to a new level at the GOP debate in Detroit, Michigan.

The billionaire businessman traded below-the-belt blows with fellow Republican contender Marco Rubio following Trump's success on Super Tuesday.

Donald Trump emphatically responded to attacks from Marco Rubio earlier in the campaign, in which Rubio had accused Trump of having "small hands", and insinuated he must also have small genitalia.

Did Donald Trump really talk about his private parts? Trump said. "And [Marco Rubio] referred to my hands, 'If they're small, something else must be small.' I guarantee you there's no problem". And neither Rubio nor Cruz wanted to give Trump any wiggle room on that question.

"Nominating Donald would be a disaster", Cruz said.

Kasich was low-key and stayed out of the verbal fight.

The party's 2008 White House nominee, Senator John McCain, said he shared Romney's concerns and those expressed by top national security experts, calling Trump's statements "uninformed and indeed dangerous". "And, people say everywhere I go, 'You seem to be the adult on the stage'". "For example, four years ago, he endorsed Mitt Romney for president".

He then accused the 2012 Republican presidential nominee of being a "failed candidate" and "choke artist" who disappeared at the end of the election.

"Obviously he wants to be relevant; he wants to be back in the game", Trump said.

Romney's speech, an unheard-of attack by a former party candidate for the nation's highest office, signalled a state of desperation inside the party establishment.

"I've supported Democrats and I've supported Republicans, and as a businessman I owed that to my company, to my family, to my workers, to everybody to get along", he said.