Mitt Romney, perhaps regretting his decision not to enter the race this year, said there was something sketchy in Donald Trump’s taxpaying history. In an interview with Fox News, the 2012 Republican presidential nominee said, “We have good reason to believe that there’s a bombshell in Donald Trump’s taxes.”

Apparently expecting that a charge this serious would be accompanied by evidence, host Neil Cavuto asked why he thought that. Romney, however, could only offer speculation.

“Either he’s not as 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 been doing,” Romney said.

Trump, who never met an attack he couldn’t parry, said Wednesday that there were no secrets. “There is no bombshell at all other than I pay a lot of tax and the government wastes the money,” he told CNN’s Anderson Cooper. “The reason Mitt Romney brings it up is because he lost in the last election, and lost very badly.”

Romney didn’t just lose the last election; he lost, at least in part, due to attacks from Democrats on this very issue. At-the-time Senate Majority Leader Harry Reid irresponsibly claimed that Romney was not paying any taxes, an accusation that led the former governor to release five additional years of tax history. When Reid’s claims were proven false, the Democrat shrugged it off, saying that he didn’t have any regrets. For Romney to now throw that attack on Trump is a sign of questionable character, desperation, or both.

“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 later on Twitter.

It’s not exactly obvious what would be deemed a “bombshell” by those who might otherwise vote for the New York real estate magnate. That he’s less rich than he claims? It’s hard to imagine that revelation swinging even a single vote away from him, unless the difference is substantial; i.e., he’s dead broke. One look at the skyline of Manhattan is enough to prove that the man is successful.

On the question of philanthropy, look at Ted Cruz. His tax returns revealed a very stingy record of charitable donations, at odds with his evangelical ideology. But even though Mike Huckabee tried to make it a thing, it barely made a blip on the radar.

As the frontrunner in both the polls and the actual results thus far, it may be time for Trump to start practicing restraint. Romney is a big name, but he’s not running for president. Trump has succeeded with his devastating attacks, but he might do better now to ignore the peanut gallery and focus on the tough work ahead.