Democratic presidential candidate Sen. Bernie Sanders, I-Vt., speaks during a rally in Warren, Mich., Saturday, April 13, 2019. Associated Press/Paul Sancya

Sen. Bernie Sanders sent a scathing letter to the influential Center for American Progress on Saturday, accusing the think tank of smearing progressive candidates.

Sanders complained in the letter about coverage from the organization's news outlet, ThinkProgress, which produced content critical of Sanders, Sen. Elizabeth Warren, and Sen. Cory Booker.

CAP responded Sunday by arguing ThinkProgress is an editorially independent news outlet that should not be censored by political leaders.

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Sen. Bernie Sanders of Vermont dressed down an influential liberal think tank in a scathing letter on Saturday, accusing the Center for American Progress of smearing progressive candidates and threatening to "reconsider what kind of partnership we can have."

The letter, which was obtained by The New York Times, lashed out at CAP and its advocacy arm - the Center for American Progress Action Fund - and accused them of playing a "destructive role" in the Democratic presidential campaigns.

Sanders' letter outlined a litany of complaints about coverage from ThinkProgress, a news outlet run by the action fund. He accused the website of publishing negative articles targeting himself and other 2020 contenders, such as Sens. Elizabeth Warren and Cory Booker.

ThinkProgress recently published a video accusing Sanders, who recently acknowledged he is a millionaire, of hypocrisy, given the decades he has spent in public life railing against wealth inequality.

Read more: Bernie Sanders is running for president in 2020. Here's everything we know about the candidate and how he stacks up against the competition.

Incensed by the coverage, Sanders even suggested that "corporate money" was influencing ThinkProgress' content.

"I and other Democratic candidates are running campaigns based on principles and ideas and not engaging in mudslinging or personal attacks on each other," he wrote. "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 a general election."

Sanders then threatened to "reconsider" his partnership with CAP if their "actions evolve in the coming months."

"This counterproductive negative campaigning needs to stop. The Democratic primary must be a campaign of ideas, not of bad faith smears," he wrote. "Please help play a constructive role in the effort to defeat Donald Trump."

The letter was reminiscent of the infighting that erupted within the Democratic party during the 2016 campaign when progressives who supported Sanders complained of being stifled and overlooked by the Clinton-affiliated establishment.

Read more: Sanders' supporters haven't forgotten their beef with Clinton - and it's an ugly problem for the DNC in 2020

Sen. Bernie Sanders and Hillary Clinton at a joint appearance in New Hampshire. AP Photo/Andrew Harnik

CAP dismissed Sanders' complaints in a statement on Sunday, arguing that ThinkProgress is an editorially independent publication that should not be censored by the likes of CAP executives or even 2020 candidates.

"Indeed, ThinkProgress publishes articles with which we disagree," Danielle Gibbs Léger, CAP's executive vice president for communications and strategy, said in a statement. "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."

ThinkProgress' editor in chief, Jodi Enda, also said in a statement that CAP and CAP Action had nothing to do with the content Sanders took umbrage with.

"Like journalists at other news organizations, ThinkProgress reporters and editors work hard every day to produce quality journalism," Enda said. "Political leaders should not be able to muzzle or stop critical coverage."

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