PHILADELPHIA - The wealthy Democratic donors, many of them executives who run complex businesses, know firsthand how revealing tax returns can be. Perhaps that's why they can't stop talking about Republican nominee Donald Trump's refusal to release his.

In their suites at the Ritz Carlton hotel, where many are staying during this week's Democratic convention, and at its auxiliary swanky parties, the supporters of Hillary Clinton are sounding the alarm about Trump's break with decades of presidential campaign tradition.

Clinton put out eight years of recent tax filings last summer, and they lament that voters don't seem to understand why Trump's refusal to do the same matters.

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Democratic talk of the taxes spilled onto the convention stage Wednesday night. Vice presidential candidate Tim Kaine, mocking Trump, said, "Believe me, there's nothing suspicious in my tax returns. Believe me!" The crowd laughed.

There's even a literally a bounty for the Trump documents.

Moishe Mana, a top fundraiser for Clinton, has offered to give $1 million to the charity of Trump's choice if he makes them public. He joins an unnamed Republican donor working with Clinton ally David Brock who has made a similar offer of $5 million.

"Through his financial documents, we are trying to break into the image that he's portraying to the American people," said Mana, a real estate developer in Miami. "He says he's a successful businessman who wants to do for the country what he did for his company. Well, go ahead, show me the money."

Trump is unmoved. The billionaire owner of the Trump Organization, an international development company, says the Internal Revenue Service is reviewing his most recent returns and that he'll release them once that audit is complete.

He reiterated that plan at a news conference Wednesday in Doral, Florida. Asked when he would put out the documents, he said: "I don't know. Depends on the audit."

There's no telling whether that would happen before Election Day, but the IRS says there's no legal reason Trump can't make the tax returns public even as they are under review.

The issue has flared up in recent days, in the wake of the hack of emails at the Democratic National Committee that the Obama administration said Wednesday was almost certainly the work of Russia. The group WikiLeaks released the emails on the eve of the convention, a leak its leader Julian Assange has said was timed to inflict political damage on Clinton.

Trump said Wednesday that he has no ties to Russia whatsoever, but that hasn't stopped Democratic donors in Philadelphia from saying that in the absence of Trump's tax returns, voters are left to wonder whether there are undisclosed financial ties between Trump and foreign entities.

"Think of what's gone on just this week and connect the dotted lines," said top Clinton donor J.B. Pritzker, a billionaire venture capitalist in Chicago. "I'm not sure what's going on, but it sure doesn't look good. The question is who his investors are, and whether there are any in China or Russia that are affecting his personal income."

Mana also wants that answered. If Trump's elected president, he said, "how much in debt would we be to other countries? This is about the security of the United States. We have the right to make sure he's not in debt to other countries."

While information about Trump's debts has been made public in personal financial disclosures filed with federal election regulators, the Democratic donors say access to his taxes might shed light on previously unknown business arrangements. The returns would also detail for the first time how much he pays in income tax and how much he gives to charity.

"He is obfuscating in order to avoid being discovered as a liar," Pritzker said.

The 2012 GOP nominee, Mitt Romney, resisted putting out his 2011 tax return until the September just before the election, after being pressed for months about doing so. The documents showed he paid an effective tax rate of 14.1 percent, far lower than the average person, spawning days of bad headlines.

Other presidential candidates, including Republican Texas Sen. Ted Cruz, have been dinged for not giving much to charity.

Bill and Hillary Clinton paid an overall federal tax rate of 31.6 percent between 2007 and 2014, her returns showed. In 2014, they donated almost 11 percent of their income to charity.

In addition to blaming the IRS audit, Trump has said in interviews that it might not make political sense for him to put out his returns.

Romney's returns were "a tiny peanut compared to mine," Trump said on "Meet the Press" in an interview that aired last Sunday. There was little controversial in the Romney documents, he said.

Yet the media "made him look bad," Trump said. "In fact, I think he lost his election because of that."