(CNN) Sen. Bernie Sanders has accused a leading liberal think tank, founded and run by longtime Hillary Clinton allies, of orchestrating attacks on him and two other 2020 Democratic presidential candidates.

In a letter provided to CNN by his campaign, Sanders addressed the board of the Center for American Progress and CAP Action Fund on Saturday, alleging that its activities are playing a "destructive role" in the "critical mission to defeat Donald Trump." Sanders cited two posts about him by ThinkProgress, a website run by CAP's political arm, and past pieces focused on Sens. Elizabeth Warren and Cory Booker.

The exchange threatens to shred an already frayed public détente between the wider circles surrounding both Sanders and Clinton, who fought a bitter 2016 presidential primary that still looms large in the minds of many Democrats -- if only because they fear a divisive replay in 2020.

CAP, founded in 2003 by John Podesta, who was former President Bill Clinton's final chief of staff and Hillary Clinton's 2016 campaign chairman, and its top officials have often been accused by progressives loyal to Sanders of seeking to undermine his political agenda -- debates that frequently blow up on social media platforms like Twitter.

Saturday's letter from Sanders follows ThinkProgress' publication of a video last week suggesting Sanders had altered his message, which has long singled out "millionaires and billionaires," after becoming a millionaire himself.

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