Sen. Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.) is accusing a liberal think tank of "using its resources to smear" him and other candidates seeking the Democratic nomination for president in 2020.

Sanders wrote in a letter to the Center for American Progress (CAP) on Saturday that the group has unfairly criticized him, Sen. Elizabeth Warren Elizabeth WarrenHarris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda Judd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report MORE (D-Mass.) and Sen. Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (D-N.J.).

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“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," Sanders continued.

Sanders wrote in the letter, which was obtained by The New York Times, that the group recently published an article on ThinkProgress that criticized him "for my appearance and for the income I earned from writing a book."

"Then, a day later, you published a video that dishonestly attacked me for hypocrisy in my effort to address income inequality in America," he said.

He added that the group ”unfairly targeted” Warren in 2017 and 2018 as well as Booker earlier this year for teaming with Sanders on a prescription drug bill.

“I will be informing my grassroots supporters of the foregoing concerns that I have about the role CAP is playing. Should your actions evolve in the coming months, I am happy to reconsider what kind of partnership we can have," Sanders wrote.

CAP responded to Sanders that ThinkProgress is editorially independent from the think tank.

"The Center for American Progress is a research institution focused on ideas and policy. 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," the Center for American Progress (CAP) said in a statement.

"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," the group added.

-Updated 2:52 p.m.