An email made public by the State Department Thursday suggests a major Democratic donor may have had second thoughts about his decision to back then-Sen. Barack Obama over Hillary Clinton in the 2008 Democratic primary.

The email, which was released alongside thousands of pages of Clinton's private emails, indicated billionaire activist George Soros had told a Clinton insider that he "regretted his decision in the primary" just as President Obama's reelection campaign was heating up in 2012.

Neera Tanden, a longtime Clinton friend and president of the Center for American Progress, recounted to Clinton the scene from a dinner in May 2012 during which she had shared a table with Soros.

"I told him I worked for you in the primaries and he said he's been impressed that he can always call/meet with you on an issue of policy," Tanden wrote to Clinton.

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