Fox News contributor George Will says GOP presidential nominee Donald TrumpDonald John TrumpFormer Joint Chiefs chairman: 'The last thing in the world we need right now is a war with Iran' Pence: 'We're not convinced' downing of drone was 'authorized at the highest levels' Trump: Bolton would take on the whole world at one time MORE will not release his tax returns because they may show "he is deeply involved in dealing with Russia oligarchs."

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The claim — which Will could not support with any tangible proof — was made to Bret Baier on Fox News's "Special Report" live from the Democratic National Convention in Philadelphia on Monday night. The topic was raised after some Democratic Party officials, including presumptive Democratic presidential nominee Hillary ClintonHillary Diane Rodham ClintonTrump: I'd rather run against Biden Democrats target Florida Hispanics in 2020 George Conway says new rape allegation against Trump 'is more credible' than Juanita Broaddrick MORE's campaign manager, Robby Mook, attempted to connect the Republican presidential nominee to a leak of Democratic National Committee (DNC) emails.

The Friday release by WikiLeaks of those emails, which appear to show an effort by DNC officials to lead a campaign against Clinton's primary rival, Sen. Bernie SandersBernie Sanders Hickenlooper, Bennet bring deep ties to 2020 debate stage Sanders: Trump taking credit for easing tensions he helped create 2020 Democrats vow to expand abortion access at Planned Parenthood event MORE (I-Vt.), led to the resignation of DNC Chairwoman Debbie Wasserman Schultz, who will step down after the convention.

"Both the campaign chair and anybody you talk to, including Sen. [Chris] Murphy [D-Conn.] would not go down that road once pressed on the connection between Russia and the Trump campaign," said Baier. "But they have thrown it out there. George?"

"Well, it's the sort of thing we might learn if we saw the candidate's tax returns," Will responded. "Perhaps one more reason why we're not seeing his tax returns — because he is deeply involved in dealing with Russian oligarchs and others. Whether that's good, bad or indifferent, it's probably the reasonable surmise."

Will, 75, declared last month he was leaving the Republican Party as a result of Trump's rhetoric, stating, "I decided that, in fact, this was not my party anymore."

The former editor of National Review was a political commentator for ABC News for 42 years before joining Fox News in 2013.