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A former head of the IRS is calling on Donald Trump to release his tax returns, saying that there is no legitimate reason to withhold them.

“When Americans file their tax returns each year, they declare under penalty of perjury that to best of their knowledge they are true, correct and complete,” says Mark Everson, who served as IRS Commissioner during the Bush Administration.

“Trump is no exception,” Everson tells NBC News, adding, “Trump either stands by his returns – which he previously promised to release – or he doesn’t.”

The call from a top ranking Republican tax expert comes as Mitt Romney is criticizing Trump on his taxes, business record and qualifications to be president.

Trump insists he will not release his returns during an audit.

"As soon as the audits are finished, I have no problem,” he said. For public information about his finances, Trump points to the financial disclosures he already submitted under federal election law.

Everson tells NBC, however, that an audit does not preclude Trump from releasing the returns.

“He can release his returns in the morning if he wants to -- nothing is stopping him from doing that,” he says.

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Everson notes that normal taxpayers provide tax returns to companies even during an audit, because the original return reflects a taxpayer’s factual tax status from the time of the filing.

“The returns themselves are the issue -- if they’re adjusted through the audit process, fair enough, that doesn’t change what he’s filed,” says Everson.

The former Commissioner, who served four years during the Bush administration, also emphasized that presidential candidates traditionally release returns for public scrutiny.

“It would be a real rupture in our political system,” he says, if a presidential candidate simply refused “to honor that transparency” and denied voters any chance to assess their taxes.

Everson, who declared his candidacy for the Republican presidential nomination and withdrew in November, says he does not know why Trump is refusing to release his returns. “He obviously has done a calculus that’s in his interest to do so,” Everson says.

Republican pressure on Trump for transparency is only mounting. Marco Rubio and Ted Cruz have both released their returns, and the two campaigns tell NBC News that Trump's taxes will be an issue at tonight's debate.

Signing a recent tax return- isn't this ridiculous? pic.twitter.com/UdwqF4iZIZ — Donald J. Trump (@realDonaldTrump) February 25, 2016

Trump's rivals are trying to paint him as a con-man with something to hide.

Cruz is openly suggesting Trump's evasiveness may stem from his "business dealings with the mob, with the mafia," as he told Meet the Press on Sunday. The senator even implied while campaigning in Georgia this weekend that Trump could be guilty of tax fraud.

Another former senior IRS official, who requested anonymity to discuss the issue, said Trump’s financial profile suggests he may be paying a very small amount of taxes, or his yearly income may be lower than Trump wants to acknowledge.

“This may not be a political judgment,” the former official said, “but an ego thing, about his wealth, where the taxes suggest he’s not as rich” as his statements have suggested.

While the current IRS has officially stated that an audit does not preclude a taxpayer from releasing tax returns, several tax lawyers have said withholding such information during an audit is prudent.

"I would characterize it as a prudent measure," says tax attorney Bradley Dorin. "I think it’s pretty straightforward, if he were my client, I would advise him to do the very same thing."

Another veteran tax lawyer says as a matter of exposure, a public release could open a client up to more second-guessing of his numbers by other people.

"If you report an expense - say contract labor - and suppose someone who did the labor might come out and say it was a different amount," said the lawyer, who did not want to be identified discussing a political dispute.

"If someone came forward, and said, 'that number is wrong, I know the real number,'" the lawyer said, that could create legal and political headaches.

Another tax lawyer says secrecy is a legitimate legal tactic – it shields a client from any possible liability -- but a candidate’s release of tax returns may be expected to go beyond the legal minimum.

"I think the main reason Trump's doing it is for political reasons," says the tax lawyer, who requested anonymity to discuss politics, "but I do think there are situations where it would be legally advisable."

What reason is there to believe @realDonaldTrump is being audited? https://t.co/AguZVCSRHh — stuart stevens (@stuartpstevens) March 3, 2016

For his part, Trump has leaned on the legal rationale since the tax issue arose at last week’s Republican debate. On Sunday, he emphatically told Fox News, “No lawyer would ever allow you to release a tax return while you're being audited.”