Mitt Romney is going after Republican presidential candidate Donald TrumpDonald John TrumpZuckerberg launches public defense of Facebook as attacks mount Trump leaning toward keeping a couple hundred troops in eastern Syria: report Warren says making Israel aid conditional on settlement building is 'on the table' 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.
 

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"I think there's something there. 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 to the disabled like he's been telling us he's doing."
 
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.

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 Mason ReidTrump thanks Reid for warning Democrats not to underestimate him Reid warns Democrats not to underestimate Trump Harry Reid predicts Trump, unlike Clinton, won't become more popular because of impeachment 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 Antonio RubioTurkey says soldier killed despite cease-fire in Syria White House staggers after tumultuous 48 hours Erdoğan got the best of Trump, experts warn MORE and Ted CruzRafael (Ted) Edward CruzHillicon Valley: GOP lawmakers offer election security measure | FTC Dem worries government is 'captured' by Big Tech | Lawmakers condemn Apple over Hong Kong censorship Lawmakers condemn Apple, Activision Blizzard over censorship of Hong Kong protesters The Hill's Morning Report — Trump's impeachment jeopardy deepens 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.