On Saturday, the Center for American Progress (CAP), one of the country’s leading liberal think tanks, received a letter from the current favorite for the Democratic nomination in 2020. “Dear members of the Board,” it began. “I write to express my deep concern and disappointment with the role that the Center for American Progress and its affiliated Action Fund arm are playing in the critical mission to defeat Donald Trump.”

The letter’s author was none other than Sen. Bernie Sanders, I-Vt., who blistered CAP’s affiliate site ThinkProgress for a recent op-ed that disparaged his physical appearance, as well as a separate video that suggested that his calls for economic redistribution were fundamentally hypocritical—this because his book royalties have made him a millionaire. Sanders also took aim at the organization’s president, Neera Tanden, for preaching solidarity while “belittling progressive ideas,” openly speculating that “corporate money … is inordinately and inappropriately influencing the role [CAP] is playing in the progressive movement.”

Sanders’ letter was likely years in the writing. As the New York Times’ Kenneth P. Vogel and Sydney Ember observe:

Mr. Sanders’s criticism of the Center for American Progress, delivered on Saturday in a letter obtained by The New York Times, reflects a simmering ideological battle within the Democratic Party and threatens to reopen wounds from the 2016 primary between him and Hillary Clinton’s allies. The letter airs criticisms shared among his supporters: that the think tank, which has close ties to Mrs. Clinton and the Democratic Party establishment, is beholden to corporate donors and has worked to quash a leftward shift in the party led partly by Mr. Sanders.

CAP has been an inextricable part of Democratic politics since Hillary Clinton’s future campaign chairman, John Podesta, founded the organization in 2003. As the Times’ Vogel and Ember reveal, the think tank helped draft policy proposals that Barack Obama would ultimately use for his 2008 campaign, and later served as a feeder of sorts for his administration. The think tank was expected to serve the same function for Clinton had she prevailed in the 2016 election; Podesta and Tanden were among her top choices for White House chief of staff.

Last year, CAP partnered with the American Enterprise Institute on a project titled “Defending Democracy and Underwriting the Transatlantic Project,” ostensibly to study how we can preserve free and open societies amid a rising tide of authoritarianism. In the process, the think tank donated $200,000 to an organization whose most prominent members include neoconservative Bill Kristol and racial eugenicist Charles Murray, leading some in progressive circles to wonder “why is the Center for American Progress betraying the left?”

In a 2013 investigation for The Nation, Ken Silverstein revealed that CAP has accepted contributions from the likes of Goldman Sachs, Wells Fargo, Coca-Cola, Citigroup, BlueCross BlueShield and weapons manufacturer Northrop Grumman, not to mention hundreds of thousands of dollars from the United Arab Emirates—among the world’s most brutal and oppressive regimes. Tanden, who regularly rails against Russian President Vladimir Putin as a “proto-fascist,” has herself come under criticism online for cozying up to such strongmen as Israel’s Benjamin Netanyahu and India’s Narendra Modi.

ThinkProgress claims to enjoy editorial independence from CAP, and Editor-in-Chief Jodi Enda asserted as much in a statement to the Times, insisting that the group and its action funds “had nothing to do with the article or video about Senator Sanders or articles related to any other political leader.” She also added that the site “will not take sides in the Democratic primary.” Tanden, for her part, has called the matter “unfortunate,” claiming that “we share the goal of unity.” Still, it’s not difficult to imagine the think tank has a vested interest in preventing Sanders from becoming the party’s nominee. The self-proclaimed democratic socialist is unlikely to rely on CAP as Obama and Clinton have before him, and so the threat he poses to Tanden and her ilk is existential: their grip on power, within the party and the country, is at stake.

Last December, as rumors swirled about Beto O’Rourke’s possible run for president, Tanden accused several prominent Sanders backers of coordinating attacks against the moderate Texas official. Responding to an op-ed from the Washington Post’s Elizabeth Bruenig, she tweeted: “Feels a bit orchestrated and clearly they are worried.” But as Sanders cements his frontrunner status, her words increasingly look like projection. And after watching Democrats lose the White House in grotesque fashion to a glorified game show host, he refuses to let bad-faith criticism from purportedly liberal media go unanswered.

Sanders’ supporters have not forgotten 2016’s contentious primary, with ample evidence that Democratic elites helped seal his defeat. While his campaign says he will honor his pledge not to go negative in ads for 2020, the Vermont senator nonetheless appears determined to prevent history from repeating itself.

Update: Tanden has since issued the following statement: