Mitt Romney is going after Republican presidential candidate Donald TrumpDonald TrumpTrump: I was 'psyched to terminate' NAFTA Trump: 'Major, major' conflict with North Korea possible Cohn: People 'wasting time' calling for Trump's tax returns MORE for not yet releasing his taxes.
"Frankly, I think we have a good reason to believe that there's a bombshell in Donald Trump's taxes," the 2012 GOP nominee told Fox News on Wednesday.
Romney said it's suspicious that every time Trump is asked about his taxes, he "dodges" and "delays."
He added that Trump likes to "tell people how well he's done" but has not yet let people look at his taxes.
"We're talking about taxes already filed, back taxes. And my back taxes, when I ran in 2012, my back taxes I put out in January of 2012," he said.
Trump shot back on Twitter, mocking Romney’s defeat in the 2012 election.
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— Donald J. Trump (@realDonaldTrump) February 24, 2016
Romney said he is sensitive to the tax issue because it affected his own run for the White House.
"They were all over me for my taxes, became a big issue, but I put my taxes out in January of 2012, and that gave people a chance to digest it and decide whether I was going to be the nominee or not," he said.
Senate Minority Leader Harry ReidHarry ReidDraft House bill ignites new Yucca Mountain fight Week ahead: House to revive Yucca Mountain fight Warren builds her brand with 2020 down the road MORE (D-Nev.) lobbed similar attacks at Romney during his 2012 campaign.
Reid insinuated that Romney wasn't releasing his tax returns because they showed him paying a very low effective tax rate.
Romney was criticized during his campaign for not releasing his pre-2010 tax returns. He ultimately gave two years of information and a summary of returns dating back to 1990.
Reid said last year he didn't regret the attacks and he did what he "felt" he had to.
Romney also criticized Trump's rivals Marco RubioMarco RubioOvernight Defense: Commander calls North Korea crisis 'worst' he's seen | Trump signs VA order | Dems push Trump to fill national security posts What’s with Trump’s spelling mistakes? Boeing must be stopped from doing business with Iran MORE and Ted CruzTed CruzNet neutrality fight descends into trench warfare Secret Service: No guns at Trump NRA speech Cruz: Breaking up 9th Circuit Court ‘a possibility’ MORE for not releasing their taxes yet.
Romney said the candidates should get their taxes out in the public so voters can see whether there are any issues, noting they will give voters a sense of whether the candidates have been telling the truth about themselves.
"The voters have a right to see those tax returns before they decide who our nominee ought to be," he said.
"People have a right to know if there's a problem in those taxes before they decide who our nominee is going to be."
Updated at 6:23 p.m.