Donald Trump once again has said he won't release his tax returns because they're under audit.

He indicated in an Associated Press story that he would do so after the audit ends, which may not happen before November.

Yet here's the thing: There's nothing preventing Trump from releasing his tax returns now. Not the IRS, not the law, not even precedent.

Turns out Richard Nixon publicly released his tax returns while they were under audit in 1973, at the height of the Watergate investigation.

But Watergate wasn't the reason, according to tax historian Joe Thorndike.

In the summer and fall of that year, "Nixon was engulfed by a controversy over his personal taxes. An outsize charitable donation was the proximate cause, but the scandal expanded to include numerous issues with the returns Nixon had filed between 1968 and 1972," Thorndike noted in a soon-to-be-published paper.

Interestingly it was that very tax controversy -- not Watergate -- that gave rise to one of Nixon's most famous quotes, Thorndike found. "People have got to know whether or not their President is a crook," Nixon told reporters in November 1973. "Well, I am not a crook."

He even went so far as to invite the Joint Committee on Taxation to also examine his returns. It did, and found Nixon owed another $476,431 -- or about $2.5 million in today's dollars. The IRS came to a similar conclusion.

Related: Trump says he can't release tax returns because of audits

But back to Trump.

It's certainly understandable why someone -- a public figure with a complicated financial life no less -- would not want to release his tax returns, especially when they're under audit. After all, why invite the added scrutiny from tax experts coast to coast?

The problem with that argument, though, is this: Trump is applying for the biggest and arguably most important job in the world.

And a requirement of that job in modern times is transparency.

While not required by law, full tax return disclosures have come to be expected of presidential nominees as well as every sitting president, audit or no.

"Americans have good reason to worry about the tax behavior of high-ranking officials. After all, when Nixon was actually serving as president, he was claiming unjustified deductions and failing to report all his income," Thorndike noted.

When asked to comment on the fact that Nixon had released his returns under audit, a Trump campaign spokesman said, "Mr. Trump is undergoing a routine audit. The documentation of this is available on our website."

-- CNNMoney's Cristina Alesci and Laurie Frankel contributed to this report