Donald Trump: Mitt Romney is a 'failed candidate'

US presidential contender Donald Trump has hit back at fellow Republican Mitt Romney's scathing criticisms of his abilities, mocking him as a "failed candidate" and a "choke artist".

"I backed Mitt Romney," said Mr Trump, addressing a crowd in Maine. "You can see how loyal he is, he was begging for my endorsement.''

He said Mr Romney ran "the worst campaign ever" and should have beaten Barack Obama in 2012.