In the latest sign of mounting strain on medical workers, nearly 300 primary care physicians employed by Mass General Brigham have notified federal authorities that they want to join a union, citing "burnout" and the "corporatization of medicine" at the state's largest health system. The doctors comprise the "vast majority" of primary care physicians employed by Massachusetts General Hospital and Brigham and Women's Hospital, according to Dr. Michael Barnett, a primary care doctor at the Brigham and associate professor of health policy and management at the Harvard T.H. Chan School of Public Health. Barnett, one of the organizers of the effort, said the physicians signed cards saying they wanted to join the Doctors Council of the SEIU, Local 10MD. The cards were delivered to the National Labor Relations Board along with a petition to hold a union election.