Mitt Romney is going after Republican presidential candidate Donald TrumpDonald John TrumpIvanka Trump pens op-ed on kindergartners learning tech Bharara, Yates tamp down expectations Mueller will bring criminal charges Overnight Cybersecurity: Equifax security employee left after breach | Lawmakers float bill to reform warrantless surveillance | Intel leaders keeping collusion probe open 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 ReidChris Murphy’s profile rises with gun tragedies Republicans are headed for a disappointing end to their year in power Obama's HHS secretary could testify in Menendez trial 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 RubioOvernight Defense: Tillerson, Trump deny report of rift | Tillerson says he never considered resigning | Trump expresses 'total confidence' in secretary | Rubio asks Army to kick out West Point grad Rubio asks Army to kick out West Point grad with pro-communist posts GOP establishment doubts Bannon’s primary powers MORE and Ted CruzRafael (Ted) Edward CruzWhatever you think the Alabama special election means, you’re probably wrong This week: Congress gets ball rolling on tax reform Week ahead: Senators work toward deal to fix ObamaCare markets 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.