Over the weekend, Sanders sent the Center of American Progress — a powerful Democratic think tank that’s closely aligned with the Clintons — a scathing letter, hitting several negative articles and videos published by the group’s media arm, ThinkProgress, that he says will make it easier for President Trump to win in 2020.

“I and other Democratic candidates are running campaigns based on principles and ideas and not engaging in mudslinging or personal attacks on each other,” Sanders wrote in the letter, first obtained by the New York Times.