The San Francisco Chronicle has obtained a letter by Andy Stern, the leader of the massive Service Employees International Union, that charges Oakland health care union giant and longtime rival Sal Rosselli with abusing his role as a union leader. The letter appears to be a prelude to an effort by Stern to strip Rosselli of his leadership of the 120,000-strong United Healthcare Workers and install his own allies. For years, Rosselli has accused Stern of compromising workers’ rights in order to cut deals with health care companies and grow the union’s membership. SF Weekly columnist Matt Smith has done some stellar work charting this fight, which seems to be coming to a head. This June, Stern and Rosselli will come face-to-face at a national SEIU meeting in Puerto Rico; don’t forget to dust off those brass knuckles, boys.