"I support you, Donald Trump. Now release your tax returns," Rep. Mark Sanford (R-S.C.) wrote in an op-ed published Sunday in the New York Times.

"The presidency is the most powerful political position on earth, and the idea of enabling the voter the chance to see how a candidate has handled his or her finances is a central part of making sure the right person gets the job," the former governor said.

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In May, about 64 percent of Americans agreed with Sanford — including 41 percent of Republican registered voters, according to Washington Post-ABC News polling. About 29 percent of registered Republicans "strongly" thought he should.

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You could argue, based on those numbers, that Trump has no political incentive to do anything with his taxes. And Sanford has been a critic of Trump in the past. But as his op-ed indicates, he's still planning to vote for him. So are 83 percent of registered Republican voters, according to an August Washington Post-ABC News poll. (Though we are obligated to point out that 83 percent is a low number for recent presidential candidates.)

But here's where Trump's tax issue ventures into potentially tricky political territory for him: Almost 60 percent of independents think Trump should release his tax returns — and almost all of them feel strongly about it.

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Having high-profile Republicans pile on in a New York Times op-ed doesn't make it easier for Trump to let the issue of his tax returns fade into the background with that crucial voting bloc. It's true: There's no rule that presidential candidates have to release their tax returns. But every major presidential candidate since 1976 has released them. President Richard Nixon also released his tax returns in 1973 while he was under audit. Trump has said he will release them when he's done being audited, but that would probably be after the election.

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But not releasing his returns risks leaving voters to imagine the worst. Theories include: He has Russian ties. He did not pay any taxes. He has mob ties. He donates no — or very little — money to charity. He's not as rich as he says he is.

Hillary Clinton's campaign has tried to seize the narrative Trump is leaving wide open. Ahead of a foreign policy speech Trump gave Monday, her campaign called on his to reveal any connections to Russia. That comes off a New York Times report Monday purporting to show that Trump's top aide, Paul Manafort, helped coordinate the campaign of a pro-Russian political party in Ukraine a decade ago. But the absence of Trump's tax returns also leaves a giant question mark.

And that's the problem for Trump. Many of the theories we listed above could in some part be rooted in investigations into what we do know about his finances. The Post's David Fahrenthold spent weeks trying to track down charities Trump said he has donated "millions" to and could find only $10,000 in donations. Forbes, Bloomberg News and others have tried to calculate Trump's net worth and found it to be less than the $10 billion he claims.

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Trump's vice presidential pick, Indiana Gov. Mike Pence (R), said Saturday that he plans to release his tax returns and that it was "proper" for Clinton to have released her taxes on Friday. It puts Trump in an awkward position when he demands President Obama's personal records.

Even Trump earlier indicated that it's a good thing for presidential candidates to do. As Sanford points out, Trump said in 2014: “If I decide to run for office, I’ll produce my tax returns, absolutely, and I would love to do that."