Donald Trump is hitting back after Mitt Romney suggested that a "bombshell" is lurking in the tax returns he has so far refused to release.
The 2012 Republican presidential nominee said on Fox News Wednesday that the contents of Trump's tax returns are the reason the billionaire developer has not released them.
But Trump soon replied on Twitter that "Mitt Romney, who totally blew an election that should have been won and whose tax returns made him look like a fool, is now playing tough guy."
He piled on the insults Thursday morning again on Twitter by calling Romney a "dope" and "one of the dumbest and worst candidates in the history of Republican politics."
"Why doesn’t @MittRomney just endorse @marcorubio already?" he wrote. "Should have done it before NH or Nevada where he had a little sway. Too late now!"
Romney had said in an interview with Fox News that Trump's foot-dragging on releasing his returns suggests he has something to hide. The former Massachusetts governor also called on Marco Rubio and Ted Cruz to release their tax returns.
Romney responded to Trump Thursday, tweeting, "Methinks the Donald doth protest too much. Show voters your back taxes, @realDonaldTrump. #WhatIsHeHiding."
Romney came under intense scrutiny in 2012 over his own tax filings.
Romney also said he believes Trump has the clearest path to the Republican nomination and that there is "a slimmer and slimmer opening" for his rivals unless the field narrows.
Trump endorsed Romney in 2012 but has been deeply critical of Romney's performance as a candidate since mounting his own run.
He added Wednesday that when Romney asked for his endorsement, "he was so awkward and goofy that we all should have known he could not win!" Romney's Twitter account did not immediately reply.