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(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.

In a tweet on Sunday, ThinkProgress editor-in-chief Jodi Enda defended the site as “editorially independent journalistic entity” and said “neither CAP or CAP Action had anything to do with the article or video related to Senator Sanders or articles related to any other political leader.”

Enda was previously employed with CNN Politics for its 2016 election coverage.

Sanders in his letter also singled out CAP’s president, a close adviser to Clinton during the 2016 campaign, accusing her of hypocrisy and untoward “personal” criticism.

“Center for American Progress leader Neera Tanden repeatedly calls for unity while simultaneously maligning my staff and supporters and belittling progressive ideas,” Sanders wrote, adding: “I and other Democratic candidates are running campaigns based on principles and ideas and not engaging in mudslinging or personal attacks on each other. Meanwhile, the Center for American Progress is using its resources to smear Senator Booker, Senator Warren, and myself, among others. This is hardly the way to build unity, or to win the general election.”

CAP responded to the letter, which was first reported by the New York Times, by insisting it did not interfere with or guide ThinkProgress’ work.

“ThinkProgress is part of CAP Action: It is editorially independent of both CAP and CAP Action and has been for years. We do not suggest, edit, approve or see their stories before publishing. And, in this particular instance, no one at CAP or CAP Action knew about this article or video’s existence before publication,” Daniella Gibbs Léger, executive vice president for communications and strategy at American Progress, said in a statement on Sunday. “Indeed, ThinkProgress publishes articles with which we disagree. We cannot and will not muzzle ThinkProgress, an editorially independent journalistic enterprise, and we believe it’s wrong for any political leader to demand it do so.”

CAP’s email to reporters also cited an article written in 2008 by current Sanders campaign manager Faiz Shakir, who worked at the think tank and was editor-in-chief at ThinkProgress before leaving for Capitol Hill, defending the site’s independence.

“In my experience over the past four years here at CAPAF,” Shakir wrote in 2008, “that editorial freedom has allowed ThinkProgress to be on the leading edge of breaking news and analysis.”

But in a reply on Twitter minutes after the CAP statement went out, Shakir said “the drafting and publishing of this post (from 2008) wasn’t ‘independent.’ Happy to talk to any journalists who want to discuss ‘editorial independence’ if that’s the debate they want to have.’”

Questions over ThinkProgress’ editorial process aren’t new. A CAP official insisted to CNN on Sunday that there was a firewall between the site and the think tank, but that it has been treated skeptically by critical outsiders.

“I regularly field calls from people angry with CAP over something Think Progress was written or posted,” the official said. “Like in this case, I always remind them that CAP has no editorial control over Think Progress.”

Former Sanders surrogate Jonathan Tasini in an email called the ThinkProgress clip “despicable” and said he initially “thought it was a Republican attack video masquerading as a Democratic production.”

CAP, Tasini added, had broken “the tacit understanding of unity between the presidential candidates who have been careful not to do anything to undercut efforts to defeat Trump.”