Fox News contributor George Will says GOP presidential nominee Donald TrumpDonald TrumpHarry Reid reacts to Boehner book excerpt: 'We didn't mince words' Man arrested for allegedly threatening to stab undercover Asian officer in NYC Trump says GOP will take White House in 2024 in prepared speech 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 ClintonClose the avenues of foreign meddling Pelosi planned on retiring until Trump won election: report Pence autobiography coming from Simon & Schuster 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 SandersHillicon Valley: Amazon wins union election — says 'our employees made the choice' On The Money: Biden .5T budget proposes major hike in social programs | GOP bashes border, policing provisions Overnight Defense: Biden proposes 3B defense budget | Criticism comes in from left and right | Pentagon moves toward new screening for extremists 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.