Mitt Romney. REUTERS/Mike Segar Former Republican presidential nominee Mitt Romney on Wednesday called on Republican candidates to release their tax returns — and speculated there could be a "bombshell" in those of frontrunner Donald Trump.

"We have good reason to believe that there's a bombshell in Donald Trump's taxes," Romney told Fox News host Neil Cavuto on Wednesday.

"What do you mean?" Cavuto asked Romney.

Romney said:

Well, 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 the disabled, like he's been telling us he's been doing.

Romney's statement came after Cavuto asked him why he had not yet made an endorsement in the Republican presidential race. As Trump has continued to march toward the nomination, Republican Party establishment members have rallied around the candidacy of Sen. Marco Rubio of Florida.

Romney said he'd like to see the taxes of each candidate.

"Donald Trump and Marco Rubio and Ted Cruz have not shown us their back taxes," Romney said. "This was an issue on my campaign."

Indeed, the Obama campaign and its Democratic allies in 2012 made Romney's taxes a signature issue. Then-Senate Majority Leader Harry Reid famously speculated that Romney didn't pay any taxes for a decade.

Romney eventually released two years' worth of returns that showed he had paid an effective rate of less than 15%. He called on the Republican candidates to do the same but zeroed in on Trump's supposed hesitance to do so.

"The reason I think there's a bombshell in there is that every time he's asked about his taxes, he dodges or delays and says, 'Well, we're working on it,'" Romney said of Trump.

Trump mocked Romney as a "tough guy" on Twitter later Wednesday.

"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," the mogul wrote.

Trump told conservative radio host Hugh Hewitt on Monday that he would release his returns "at some point, probably." He has also bristled at the notion, as Romney suggested, that any of his financial statements might show that he is not as wealthy as he has claimed.

He told Hewitt:

We'll be working on it. Everything is very much, you know, I gave my financials ahead of schedule, much ahead of schedule. I had a long time to give them, and I gave them immediately. And they were very complex, also, and very big, and they turned out to be extremely good, much better, actually, than people thought.

This photo is from 2012, when Donald Trump thought better of Mitt Romney and endorsed him. AP Photo/Gerald Herbert

Romney, who has long been critical of Trump, emerged back into the public discussion over the 2016 presidential race on Wednesday when he was quoted discussing the year's political environment.

"We're just mad as hell and won't take it anymore," he said of the electorate on Tuesday, according to The Washington Post.

"The failure of current political leaders to actually tackle major challenges, or to try at least, or to go out with proposals," he added, speaking at Babson College in Wellesley, Massachusetts.

Watch Romney's Fox appearance below: