Donald Trump greets then-Republican presidential candidate and former Massachusetts governor Mitt Romney  after announcing his endorsement of Romney during a news conference on Feb. 2, 2012, in Las Vegas. (Julie Jacobson/AP)

Former GOP presidential nominee Mitt Romney  assailed billionaire real estate mogul Donald Trump on  Wednesday afternoon for failing to disclose back taxes, repeatedly suggesting that the billionaire’s financial records may contain “a bombshell” that could damage his White House bid.

“I think we have good reason to believe that there’s a bombshell in Donald Trump’s taxes. I think there is something there,” Romney said on Fox News’ “Your World” with Neil Cavuto. “The reason I think there is a bombshell in there is because every time he is asked about his taxes, he dodges and delays.”

Romney said that he could see any number of things becoming an issue for the billionaire, from the possibility that he is not as wealthy as he says he is to controversy over the amount he has paid on his income. Romney added that Trump has failed to release his tax information despite assuring that his team would do so in short order.

“I think in Donald Trump’s case it is likely to be a bombshell,”  Romney said. “There are things that could be issues, and 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’s own tax record became a controversial issue during his 2012 White House campaign after Senate Majority Leader Harry Reid (D-Nev.) falsely claimed that Romney had not paid  taxes in 10 years. Romney finally relented after months of speculation about his personal finances and released his 2011 tax returns. President Obama’s reelection campaign used the issue to characterize the millionaire as untrustworthy and out of touch with the average American voter.

“[Trump] likes to tell people how well he’s done, why isn’t he willing to let us look at the tax return? It was an issue in my campaign. That’s why I’m so sensitive to it,” Romney said. “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 is an issue there."

The former Massachusetts governor said that Trump has the clearest path toward the nomination of any GOP contender still in the race. Many political strategists believe that Sen. Marco Rubio (Fla.) and Sen. Ted Cruz (Texas) could emerge as a viable alternative to the billionaire, but for now they remain locked in a bitter fight for second place. Romney said the window of opportunity for those candidates is closing.

On Tuesday  night, Trump won his third consecutive primary.

“I think for the other people still in the race, their path is becoming a slimmer and slimmer opening and they’re having a difficult time communicating to their supporters just how they could become the nominee,” Romney said, but added that “it’s not impossible.”

“That doesn’t seem to be happening other than Jeb Bush, who I really think put country first by saying, look, I’m going to step aside and let my supporters go to someone else who they feel is representative of the kind of views that the country needs,” Romney said.

Trump fired back on Twitter Wednesday evening, mocking the former candidate's loss to 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 tweeted.

"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 added later.