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Story highlights

NEW: Donald Trump said he will decide whether to release his tax returns and called Mitt Romney "yesterday's news"

The 2012 Republican nominee's broadside followed Trump's thumping victories in three of the first four GOP nominating contests

Romney warned Wednesday that Donald Trump's tax returns could contain a "bombshell"

Watch CNN’s Republican debate live on Thursday at 8:30 p.m. ET.

(CNN) —  

Donald Trump is striking back at Mitt Romney on Thursday, after the 2012 Republican presidential nominee said his party’s current front-runner could have a “bombshell” in his tax returns.

“Mitt Romney, who was one of the dumbest and worst candidates in the history of Republican politics, is now pushing me on tax returns. Dope!” the billionaire businessman said in a series of Tweets attacking Romney.

Trump wrote that tax returns have zero to do with a person’s net worth, and all swung at the establishment wing of the Republican party, many of whom have coalesced behind Florida Sen. Marco Rubio as an answer to Trump and Texas Sen. Ted Cruz.

“Why doesn’t @MittRomney just endorse @marcorubio already. Should have done it before NH or Nevada where he had a little sway. Too late now!”

Trump said, “I’m going to do what @MittRomney was totally unable to do- WIN!”

Romney used the same medium to respond, tweeting back at the billionaire businessman Thursday.

“Methinks the Donald doth protest too much. Show voters your back taxes, @realDonaldTrump. #WhatIsHeHiding,” Romney tweeted.

The real estate mogul first rejected Romney’s accusation out of hand in an interview with CNN’s Anderson Cooper on Wednesday, saying “there is no bombshell at all other than I pay a lot of tax and the government wastes the money.”

Romney’s biting attack hinted at clear signs of alarm in the Republican establishment at the billionaire’s tightening grip on the party’s presidential race.

“We have good reason to believe that there’s a bombshell in Donald Trump’s taxes,” Romney told Fox News, and also called on the top anti-Trump contenders Sens. Ted Cruz and Marco Rubio to disclose their tax information as well.

“Either he’s not anywhere near as wealthy as he says he is, or he hasn’t been paying taxes we would expect him to pay or perhaps he hasn’t been giving money to vets or to the disabled like he’s been telling us he’s been doing,” Romney added.

Nightcap: Mitt Romney predicts a ‘bombshell’ in Donald Trump’s taxes | Sign up

00:48 - Source: CNN
Romney predicts 'bombshell' in Trump's taxes

Other Republicans – including South Carolina Sen. Lindsey Graham – came to Trump’s defense Thursday.

“We as Republicans should not accept that this man has a tainted financial past,” Graham told CNN’s “This Hour,” adding later, “It’s clear to me he doesn’t want to show us his financial situation.”

The 2012 Republican nominee’s broadside followed Trump’s thumping victories in three of the first four GOP nominating contests, including in Nevada on Tuesday night, which have established the billionaire businessman as the party’s undisputed front-runner.

Opinion: It’s over; Trump is going to be the Republican nominee

It was also an attack steeped in irony, since Romney was on the receiving end of similar claims by then-Senate Majority Leader Harry Reid, when Democrats eviscerated him over his fortune and business record during the 2012 campaign. Reid zeroed in on his taxes, saying he hadn’t paid any in a decade without offering any evidence to support the claim.

Trump left the door open to not releasing his tax returns Wednesday.

The GOP front-runner told CNN that he will “make a determination over the next couple of months” as to whether he will release his tax returns.

Romney, who decided against a third presidential run last year after initially considering jumping in to the race, uncorked his attack on Trump, ahead of next week’s Super Tuesday contests that could further cement the billionaire’s strong front-runner status in the GOP presidential race.

Romney’s move appeared to not just be a sign of concern that Trump – after defying pundits and political logic since he launched his “outsider” campaign last summer – could actually go on to claim the Republican n