By way of explanation, Manafort cited the fact that Trump is the subject of an audit. But that's only one of a bevy of theories floating around as to why Trump is so adamantly opposed to releasing his returns. Let's go through them.

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1. He has Russian ties. On Monday night, George Will offered up this theory. "Perhaps one more reason why we're not seeing his tax returns is because he is deeply involved in dealing with Russian oligarchs and others," Will speculated in a conversation with Fox News's Bret Baier. "Whether that's good, bad or indifferent, it's probably the reasonable surmise."

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The context for Will's comment is a whisper campaign from some Democrats that Russia understands Trump would be better for its interests than Hillary Clinton and, therefore, is orchestrating the leaking of hacked Democratic National Committee emails to damage her. (Many experts think that Russia was indeed behind the hack.)

Trump -- and Manafort -- insist that isn't the case and that he has no business interests in Russia.

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2. He is being audited. I mean, maybe. Trump could, of course, release his tax returns even while he is under an audit. Richard Nixon did it in 1973.

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4. He has mob ties. The idea that Trump has links to organized crime has been kicking around for decades. But they've come much more to the fore since he began running for president. Ted Cruz, during a late February interview on "Meet the Press," brought the accusation into the presidential race: "There have been multiple media reports about Donald's business dealings with the mob, with the mafia. Maybe his taxes show those business dealings are a lot more extensive than has been reported."

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Mitt Romney, the party's 2012 presidential nominee, also offered a version of the mob theory in a May Facebook post.

It is disqualifying for a modern-day presidential nominee to refuse to release tax returns to the voters, especially one... Posted by Mitt Romney on Wednesday, May 11, 2016

5. He donates no — or very little — money to charity. Again, this is a theory with some meat on the bone. The Washington Post's David Fahrenthold had spent weeks contacting charities in hopes of tracking down the "millions" Trump says he has donated to various organizations over the past seven years. The total amount of charitable giving Fahrenthold found? $10,000. So ...

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6. He is not as rich as he says. Nailing down Trump's personal wealth is a massive challenge. He says $10 billion. Others say $3 billion. Still others insist he is worth far less than either number. Trump takes considerable personal pride in touting his net worth and using it as a blunt instrument to respond to any charge that he has ever failed in life. Tax returns are tough to fudge; they nail down exactly how much you make in a year. Just the raw numbers. It would be hard for Trump to argue his way out of a yearly salary that doesn't befit a billionaire.