In his letter to the co-chairs of Priorities’ board – former Michigan Gov. Jennifer Granholm and former Obama campaign manager Jim Messina – Brock alleged that “current and former Priorities officials were behind this specious and malicious attack on the integrity of these critical organizations.”

The letter – and Brock’s resignation – offer a rare glimpse into a network of groups upon which Democrats are relying to keep the White House and stave off increasingly robust big-money efforts on the right. The public airing of dirty laundry comes as sources say Priorities is struggling to live up to the hopes of some Clinton allies, who had argued it should aim to raise as much as $500 million to eviscerate prospective Clinton rivals in the primary and general elections.

Brock, who spent his early career in Washington as a self-described “right-wing hit man” before experiencing a political awakening and emerging as the leader of an empire of hard-hitting liberal attack groups, contends in his letter that Priorities is trying to damage his groups’ fundraising efforts, “while presumably enhancing Priorities’ own. Frankly, this is the kind of dirty trick I’ve witnessed in the right-wing and would not tolerate then. Our Democratic Presidential nominee deserves better than people who would risk the next election – and our country’s future – for their own personal agendas.”