Donald Trump's tax returns may contain a “bombshell,” according to 2012 Republican presidential nominee Mitt Romney.

Phoning into Fox News on Wednesday, Romney called for the top three Republican presidential candidates to release their tax documents — especially Trump.

Romney accused Trump of “dodging and weaving” on the issue, noting that he had been vague about when he would make the records public.

“We’re gonna select our nominee. We really ought to see from all three of these fellas what their taxes look like to see if there’s an issue there,” Romney said. “I think in Donald Trump’s case, it’s likely to be a bombshell.”

Romney, a co-founder of private equity company Bain Capital, came under severe pressure during the 2012 race to disclose his tax returns, which revealed that his tax rate was much lower than that of most Americans.

At the time, then-Senate Majority Leader Harry Reid claimed an anonymous source told him that Romney hadn’t paid taxes in a decade. PolitiFact, however, rated that a “pants-on-fire” lie.

On Wednesday, Romney said there's "no question" that Trump, who’s won three consecutive states by double-digit margins, has the "clearest path" to the GOP nomination. “I think for the other people still in the race, their path is becoming a slimmer and slimmer opening,” Romney said.

But should Trump release his tax returns, that opening could swell. “You know, Donald Trump has said he’s the best in the country for the disabled veterans and the disabled generals. Well, if his taxes show that he hasn’t made any contributions to the disabled veterans or to the disabled generally, that would be a big issue,” explained Romney, who was careful to note that he was merely speculating and had no evidence against the real-estate magnate.

“The reason that I think that 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,” Romney continued. “Hey, we’re not talking about the taxes that are coming due this year. Of course they’re working on those. They won’t be ready for months. We’re talking about taxes already filed — back taxes.”

Romney has periodically taken shots at Trump, including for his attempts to cozy up to Russian President Vladimir Putin, and his former aide Katie Packer has formed an anti-Trump super PAC.

Trump blasted Romney on Twitter later Wednesday for “playing tough guy” and ridiculed him for his 2012 loss to President Barack Obama. “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,” Trump said, before retweeting a post one minute later and reiterating Romney is “a fool.”

He also claimed Romney had acted like a fool when seeking his support in 2012. “When Mitt Romney asked me for my endorsement last time around, he was so awkward and goofy that we all should have known he could not win!” he tweeted.

The former Massachusetts governor publicized his back taxes in January 2012. “We’re now in late February, and we still haven’t seen either Donald Trump’s or Marco Rubio’s or Ted Cruz’s taxes and, frankly, the voters have a right to see those tax returns before they decide who our nominee ought to be,” Romney said.

On Feb. 14, Trump said on CBS's “Face the Nation” that he would release his tax returns "over the next three, four months." By Monday, he was telling conservative radio host Hugh Hewitt that he would release them “at some point, probably.”

Back in October, Trump tweeted images showing him signing a gigantic stack of IRS paperwork:

Signing my tax return.... pic.twitter.com/XJfXeaORbU

— Donald J. Trump (@realDonaldTrump) October 15, 2015

“I have one the world's most complicated tax returns,” Trump boasted on “Face the Nation.” “It's a massive return. And — but I will get it done as soon as I can.”