Sen. Bernie Sanders (I-VT) wrote a harshly-worded letter to a liberal think tank this weekend, accusing the organization of "using its resources to smear" him and two of his Senate colleagues currently running for the 2020 Democratic presidential nomination.

The letter, obtained and first reported by The New York Times, was delivered on Saturday to the board of the Center for American Progress and the Center for American Progress Action Fund. This comes after ThinkProgress, a news outfit that is editorially independent of CAP though a project of the think tank, published a video suggesting the Independent from Vermont had altered his language about wealth inequality in the United States after he had become a millionaire himself, due to the success of his recent book . In the video, they showed clips of Sanders at times denouncing just "billionaires" as opposed to "millionaires and billionaires," after his book his published.

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

As The Daily Beast reported, ThinkProgress did not view their work as being explicitly adversarial.

“ThinkProgress did not try to ‘go after’ Sen. Sanders—to use your words—and we will not ‘go after’ any other candidate,” Jodi Enda, the editor of ThinkProgress said. “We strive to be fair and to allow our readers and viewers to interpret the news for themselves. We are no different from other media in that regard.”

In a statement on Sunday, she said: "Political leaders should not be able to muzzle or stop critical coverage."

Sanders also expressed concern in his letter about corporate contributions tilting CAP's role in the broader resistance movement against President Trump.

“Center for American Progress leader Neera Tanden repeatedly calls for unity while simultaneously maligning my staff and supporters and belittling progressive ideas,” he wrote. “I worry that the corporate money CAP is receiving is inordinately and inappropriately influencing the role it is playing in the progressive movement.”

CAP was founded by John Podesta, an ally of the Clintons who served as 2016 chairman of the Hillary Clinton campaign. Its current president, Tanden, as well as Podesta, were previously critical of Faiz Shakir, when he was advising Sanders' 2016 bid. Shakir, who now serves as Sanders' 2020 campaign manager, had previously worked at CAP and helped launch ThinkProgress.

Sanders was also critical of previous pieces the websites published about two fellow Senators he is close with and currently running against in the Democratic primary.

“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 the general election.”

According to the Times, Tanden told Shakir that “we share the goals of unity" and offered to meet and discuss the "unfortunate" situation. Shakir did not immediately respond to a question from The Daily Beast about whether the meeting would take place.