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Story highlights

Clinton and Democrats say Trump is hiding something in his unreleased tax returns

Mike Pence will release his tax returns soon

(CNN) —  

Donald Trump is not releasing his tax returns.

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.

“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 lo