At least not yet. And if some of his advisers have their way, not ever.

Even as his running mate, Mike Pence, prepares his own much less interesting tax returns, and even as his opponent Hillary Clinton and all of her supporters rail on the issue -- and what Trump might be hiding -- Trump has stood firm.

The stated reason is an ongoing "routine audit."

"Just so you understand I'm under audit. A routine audit. And when the audits complete I'll release my return," Trump reiterated Tuesday on Fox News.

But Democrats point to a series of theories behind Trump's rationale to break with decades of major party nominee tradition -- and conversations with tax experts and officials indicate some, if not all, may have merit.

First and foremost is the idea that the audit precludes Trump from releasing his returns. This, according to tax lawyers, is not correct. Now, Trump has made clear that he's operating under the advice of his lawyers, who recommended he not release his returns. Trump's view is backed up by his campaign manager Kellyanne Conway, who, in an earlier stage of the election, when she was supporting Sen. Ted Cruz, had called for Trump to release his returns.

"I've learned since being on the inside that this audit is a serious matter and that he has said that when the audit is complete, he will release his tax returns," Conway said last month on ABC News.

Trump's own tax counsel, in a letter released in March, said the review of Trump's taxes encompasses the "2009 year and forward." A previous review of Trump's returns from 2002 through 2008 is complete, according to the letter, ostensibly meaning Trump could release those returns at any point should he choose. But his campaign continues to maintain that as long as an audit is ongoing, there will be no release. Trump has appeared unconcerned about the whole thing.

JUST WATCHED Chaffetz: Both Trump and Clinton need to release records Replay More Videos ... MUST WATCH Chaffetz: Both Trump and Clinton need to release records 04:46

"Nobody cares about it except some of the folks in the media. Nobody cares about it," Trump said in his Fox News interview.

Clinton's campaign, for its part, has continued to zero in on the tax issue -- with Clinton and running-mate Tim Kaine hitting Trump on the trail. Her campaign is also using the issue in in digital ads and tweets. And a "Donald Ducks" protest was arranged by Democrats in front of Trump Tower in New York.

The reason, according to Democrats familiar with the strategy, isn't exactly a state secret: 64% of Americans surveyed think Trump should release his taxes, according to a Washington Post-ABC News poll. That includes 41% of registered Republicans.

Those numbers appear to be having an affect even among Trump supporters. Rep. Mark Sanford, a South Carolina Republican, took to the opinion pages of the New York Times to call on Trump to release his returns.

"Equipping voters with more, not less, information as they pick those who run for the highest offices in our land seems, to me, a reasonable requirement for anyone aspiring to those positions," Sanford wrote. He went so far as to say his future support for the GOP nominee may depend on the eventual release of his returns.

Trump would be the first nominee not to release his returns in more than four decades. There's also precedent for the attacks on the issue serving as a potent political cudgel (see: Romney, Mitt ). But Trump and his team have made clear they aren't in a hurry to fall into line with that trend.

"We just don't see much upside," one Trump adviser told CNN.

Here are the reasons why:

Trump's tax rate is likely very low -- if not zero

Just because Trump hasn't released his returns doesn't mean there's no insight into his taxes. As a developer and a casino magnate, submitting his tax returns -- or the topline numbers from them -- is something Trump has had to do quite often throughout the course of his career. The picture it paints is one that may be contributing to Trump's reluctance to release his returns. A CNN review of the documents, most obtained through open records requests, show at least two years in the 1970s when Trump paid $0 in federal income tax.

At least one year in the next decade Trump paid nothing at all in income tax, according to New York City tax documents obtained by CNN.

An important point is this: as a real estate developer, Trump has had, over the course of his career, access to more tax breaks and loopholes than any other profession. From depreciation, to deductions and credits, and deferrals on sales, and like-kind exchanges, Trump has had ample opportunity to use the tax code to his advantage --- and he publicly acknowledges he has done just that.

"I mean, I pay as little as possible. I use every single thing in the book," Trump told supporters in Iowa in January.

Trump's charitable giving will be laid bare -- if it exists at all

There's also the issue of Trump's charitable giving. While Trump has repeatedly claimed charitable largesse, an in depth review by the Washington Post has raised questions as to whether Trump was as willing to give as he claims -- or if he gave anything at all. The details of what Trump has given individually would be shown in his tax returns.

Trump's business relationships are voluminous -- and potentially problematic

A glance at Trump's rather lengthy financial disclosure form, filed in May with the Federal Election Commission, makes clear: Trump's business empire has a long, global and rather complicated reach. As Trump's tax counsel put it in their March letter: Trump "holds interests as the sole or principal owner in approximately 500 separate entities." Those entities, Trump's lawyers said, "engage in hundreds of transactions, deals and new enterprises every year."

This, more than anything else, is the area that piques the interest of Democrats looking to attack Trump. The reason, according to one adviser to the Clinton campaign, is both the ability to play on the fear of the unknown, and tie it to the known problems Trump has had in his business career.

As Clinton told reporters on Tuesday, "The list goes on and on, the scams, the frauds, the questionable relationships with business activities that has stiffed workers, refused to pay small business. Clearly his tax returns tell a story that the American people deserve and need to know."

Trump's annual income -- not as illuminating as one might think

JUST WATCHED Clinton releases tax returns, puts pressure on Trump Replay More Videos ... MUST WATCH Clinton releases tax returns, puts pressure on Trump 02:01

The obsession over Trump's worth, both on his part and the part of outside observers, is something that would not get a definitive answer through Trump's tax returns. Trump's statement, along with those financial disclosure forms, that his "net worth is in excess of $10 billion" wouldn't be confirmed or proven false by Trump's annual returns. But the returns would provide a clearer window into Trump's individual finances -- most notably his adjusted gross income -- and shed more specific light on his finances than the disclosure form, which requires candidates only to list the values of the holdings in broad ranges.

Trump, for his part, has said repeatedly he has no problem releasing his returns -- and downplays their potential political impact.

"I don't know when that's going to be, but when the audit is complete, I'll release my returns. I have no problem with it," Trump said. "It doesn't matter."