It’s a powerful image: Blood streaming down a union organizer’s nose and splattered all over his white shirt after thugs from the Ford Motor Company attacked him and others who were distributing union fliers.

That 1937 photograph is just one of the searing scenes in “Brothers on the Line,” a new documentary about the Reuther brothers: Walter, the future United Auto Workers president standing next to the bloodied organizer, and Victor and Roy. Together they played a pivotal role in transforming the United Auto Workers into what was for decades the nation’s most powerful labor union.

Victor Reuther’s grandson Sasha Reuther features that photo prominently in the new documentary, which he directed and helped produce, to tell how the brothers built the U.A.W. and how that union helped raise living standards for not just one million autoworkers, but also for a large swath of America. The film shows the fierce struggles and sit-down strikes that led to the unionization of General Motors, Ford and Chrysler, and how the U.A.W. played a major role in underwriting the civil rights movement as well as that of Cesar Chavez and the farmworkers.