After being accused by Sen. Bernie Sanders (I-Vt.) of attempting to thwart his presidential run, the Center for American Progress, a liberal Washington-based think tank, acknowledged that a video on its affiliated website was biased.

In a statement released Monday, CEO Neera Tanden addressed the 2020 presidential hopeful’s concerns raised in a recent letter to the organization. Criticizing the group, Sanders pointed to a clip from the digital news outlet ThinkProgress, which suggested his millionaire status is antithetical to his consistent rebukes of the wealthy. The senator said the video amounted to a dishonest attack.

Tanden said CAP’s mission “is to positively engage with all political leaders about the country’s future,” and emphasized that ThinkProgress is editorially independent of it. She said ThinkProgress was “valuable as a news outlet” for that reason.

“Similarly, we at CAP can form our own opinions of their work,” Tanden said. “We believe the content of the ThinkProgress video critiquing Sen. Sanders is overly harsh and does not reflect our approach to a constructive debate of the issues.”

The video, titled “Bernie’s millionaire problem,” was posted online Thursday. It contends that he changed his rhetoric on economic status “after reaping in royalties from book sales.”

In his letter sent Sunday, Sanders said CAP and its affiliate, CAP Action, were playing a “destructive role” in efforts to defeat President Donald Trump by “using its resources to smear” not only himself but others seeking the Democratic presidential nomination. He specifically mentioned Sens. Cory Booker (N.Y) and Elizabeth Warren (Mass.).

“The counterproductive negative campaigning needs to stop,” he said. “The Democratic primary must be a campaign of ideas, not of bad faith smears.”

Sanders has openly addressed his financial situation, saying that proceeds from his 2016 bestseller, “Our Revolution,” accounts for a significant chunk of his earnings. On Monday, he released his tax returns for the last 10 years.

Defending his income at a community meeting last week in Gary, Indiana, Sanders said he “didn’t know that it was a crime to write a good book which turns out to be a bestseller.”