Sen. Bernie Sanders (I-VT) sent an angry letter to the Center for American Progress on Sunday complaining about coverage he’d received from ThinkProgress, an associated but editorially independent journalistic wing of the organization.

Referring to a ThinkProgress article about his status as a millionaire following the 2016 presidential campaign — one largely attained through book sales — Sanders said its author “criticiz[ed] me for my appearance and for the income I earned from writing my book.”

A related video about his income, Sanders said, “dishonestly attacked me for hypocrisy in my effort to address income equality in America. The video, he said, “was excitedly discussed on many conservative websites.”

After mentioning ThinkProgress articles critical of fellow Democratic presidential candidates Sens. Elizabeth Warren (D-MA) and Cory Booker (D-NJ), Sanders said he worried “that the corporate money CAP is receiving is inordinately and inappropriately influencing the role it is playing the progressive movement.”

“This counterproductive negative campaigning needs to stop,” he wrote.

ThinkProgress responded to Sanders by emphasizing its editorial independence from CAP.

Editor-in-chief Jodi Edna wrote in part: “Neither CAP nor CAP Action had anything to do with the article or video related to Senator Sanders or articles related to any other political leader.”

Neera Tanden, CAP’s president, also emphasized ThinkProgress’s independence to The New York Times. Sanders said in his letter that Tanden “repeatedly calls for unity while simultaneously maligning my staff and supporters and belittling progressive ideas.”

Perhaps ironically, as Edna herself pointed out, Sanders’ 2020 campaign manager Faiz Shakir was once editor-in-chief of ThinkProgress and defended its independence from CAP at the time. The Times reported Shakir sent Sanders’ letter to CAP leadership Saturday night.

Read Sanders’ full letter here via the Times.