As demands for reform have mounted in the aftermath of police violence in cities like Ferguson, Mo., Baltimore and now Minneapolis, police unions have emerged as one of the most significant roadblocks to change. They aggressively protect the rights of members accused of misconduct, often in arbitration hearings ... behind closed doors. And they have also been remarkably effective at fending off broader change, using their political clout and influence to derail efforts to increase accountability. When Steve Fletcher, a Minneapolis city councilman and frequent Police Department critic, sought to divert money away from hiring officers and toward a newly created office of violence prevention, he said, the police stopped responding as quickly to 911 calls placed by his constituents. It operates a little bit like a protection racket," Mr. Fletcher said of the union. Federal intervention is often one of the few reliable ways of reforming police departments. But in Cleveland, the union helped slow the adoption of reforms mandated by a federal consent decree, according to Jonathan Smith, a former U.S. Justice Department official. Mr. Smith said union officials had signaled to rank-and-file officers that the changes should not be taken seriously, such as a requirement that they report and investigate instances in which they pointed a gun. In Chicago ... a "code of silence" about misconduct was effectively "baked into" the labor agreements between police unions and the city.