Mitt Romney invokes Shakespeare to fire back at Donald Trump

The feud between Mitt Romney and Republican presidential frontrunner Donald Trump has escalated, with Romney invoking Shakespeare on Thursday to mock Trump.

Romney, the GOP nominee in 2012, has been calling for Republican candidates to release their tax returns.

On Wednesday, he speculated on Fox News that there could be a "bombshell" in Trump's.

Trump shot back, calling Romney "one of the dumbest and worst candidates in the history of Republican politics."

Romney responded by tweeting a reference to William Shakespeare's "Hamlet":

While he was running for president, Romney released two years' worth of tax returns that showed he had paid an effective rate of less than 15%. His release came after Democrats and the Obama campaign hounded Romney about his taxes during the 2012 presidential race.

Romney is now calling on all the Republican candidates to do the same, but he has zeroed in on Trump's supposed hesitance to do so.

"The reason I think there's a bombshell in there is that every time he's asked about his taxes, he dodges or delays and says, 'Well, we're working on it,'" Romney said of Trump.

Romney speculated about why Trump might be reluctant to release his tax returns:

Either he's not anywhere near as wealthy as he says he is, or he hasn't been paying the kind of taxes we would expect him to pay. Or perhaps he hasn't been giving money to the vets or the disabled, like he's been telling us he's been doing.

After Romney's comments on Fox, Trump mocked Romney as a "tough guy" on Twitter. Trump also recalled his endorsement of Romney in 2012, saying he "should have known" Romney would lose to President Barack Obama.

"When Mitt Romney asked me for my endorsement last time around, he was so awkward and goofy that we all should have known he could not win!" he tweeted.