There might be a “bombshell” revelation to be discovered in Republican presidential frontrunner Donald Trump’s tax returns, 2012 party nominee Mitt Romney said Wednesday.

He also called on the entire GOP field to release their tax returns.

“I think there’s something there," Romney said of Trump's returns, "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,” Romney, a former Massachusetts governor, told Fox News’ Neil Cavuto on “Your World.”

Trump responded later Wednesday on Twitter, slamming Romney for losing a winnable election, and calling him "awkward" and "goofy" when the governor asked for the real estate magnate's endorsement.

“The reason I think there’s a bombshell in there is because every time he’s asked about his taxes, he dodges and delays and says ‘well, we’re working on it.’ Hey, we’re not talking about the taxes that are coming due this year… We’re talking about taxes that already filed,” Romney, a multimillionaire businessman, said.

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“This was an issue in my [2012] campaign. That’s why I’m so sensitive to it,” Romney said.

Romney was pressured by President Obama’s reelection campaign in 2012 to release five additional years of his personal tax returns after the Republican’s campaign said he hadn’t paid more than 13 percent in taxes in the previous decade.

“We’re going to select our nominee. We really ought to see from all three of these fellows what their taxes look like to see if there’s an issue there,” he added, also pointing to Texas Sen. Ted Cruz and Florida Sen. Marco Rubio, “I think in Donald Trump’s case, it’s likely to be a bombshell.”

“Perhaps he hasn’t been giving money to the veterans or to the disabled like he’s been telling us he’s been doing. I think that’s the reason there’s a bombshell [in his returns],” he said.

Cavuto pressed Romney as to whether the governor was indeed questioning the amount of Trump’s declared charitable donations.

“Well, if his taxes show that he hasn’t made any contributions to the disabled veterans… that would be a big issue, and I’m not saying that’s the case,” Romney said, “I’m just saying there are things that could be issues.”

Trump, who skipped Fox News’ January GOP debate in Des Moines, Iowa, to hold a nearby rally he described as a benefit for veterans groups, has repeatedly voiced his commitment to such groups in his campaign speeches. The New York billionaire tweeted on January 29 that the veterans’ event raised $6 million.

Romney told Cavuto that voters “have a right to see [candidates’] tax returns” before deciding on a nominee. “We’re now in late February and we still haven’t seen Donald Trump’s or Marco Rubio’s or Ted Cruz’ taxes.”

“If in fact [their] taxes have some real problems, let’s get them out there and see them,” he said.

“When people decide they don’t want to give you their taxes, it’s usually because there’s something they don’t want you to see,” Romney said.

Trump submitted a financial disclosure form when he announced his presidential bid in June 2015. At the time, he declared his net worth to be about $10 billion.