Former Republican presidential candidate Mitt Romney slammed Donald Trump Wednesday for declining to release his tax returns, saying in a Facebook post the only explanation was that the documents contained a "bombshell of unusual size" about the presumptive GOP nominee.

"It is disqualifying for a modern-day presidential nominee to refuse to release tax returns to the voters," Romney wrote about the real estate mogul.

"There is only one logical explanation for Mr. Trump's refusal to release his returns: there is a bombshell in them," he continued. "Given Mr. Trump's equanimity with other flaws in his history, we can only assume it's a bombshell of unusual size."

On Tuesday, the billionaire told the Associated Press he doesn't expect to release his tax returns before November, citing an ongoing audit of his finances. 

Trump said he will release them after the audit ends, but that he wouldn't overrule his lawyers and instruct them to release his returns if the audit hasn't concluded by November.

"There's nothing to learn from them," Trump told the AP, adding that he doesn't believe voters are interested.

Trump weighed in on the issue again Wednesday, saying on Twitter: "In interview I told @AP that my taxes are under routine audit and I would release my tax returns when audit is complete, not after election!"

Trump told Fox News' Greta Van Susteren in an "On The Record" interview Wednesday it would be "meaningless" to release his returns while he is being audited.

“You learn very little from a tax return," he said. 

He also reiterated he would release after his audit was completed. 

"At the right time I'll release them," he said. 

Democratic presidential candidate Hillary Clinton used Trump's recent comment about his tax returns as a main point of her campaign rally Wednesday in Blackwood, New Jersey. 

"So you've got to ask yourself why doesn't he want to release it? Yeah, well, we're going to find out," Clinton told supporters.

The Associated Press contributed to this report.