SALT LAKE CITY -- The battle between the Mild Mannered Millionaire and the Brash Billionaire sounds like a fight out of comic books.
But it played out on Fox News and Twitter, as Mitt Romney went after Donald Trump in an interview, followed by a Trump attack on Twitter.
"Frankly, I believe we have good reason to believe that there's a bombshell in Donald Trump's taxes," Romney told Fox News' Neil Cavuto in a phone interview.
Romney speculated about Trump's tax returns, saying that Trump may not be worth the $10 billion he has claimed, or maybe he doesn't really give so much to veterans and the disabled.
In trademark fashion, Trump took to Twitter without mercy, saying Romney was the dumbest and worst Republican candidate in history in one of the seven Tweets he lobbed like verbal artillery through the early morning.
Romney responded once, with a reference to Hamlet.
"Methinks the Donald doth protest too much," he said.
But Romney came under more criticism from Senate Democratic Leader Harry Reid. Recalling 2012, Politico reports that Reid said it took "gall" for Romney to demand anyone's returns.
Reid said that Romney only released returns from the two years in which he was running for president, only providing "summaries" of previous years.
Donald Trump has said his staff is working on his returns, which are voluminous, but he has not given any indication when he plans to release them or for how many years.