Boston-based Tufts Medical Center is seeking to become a major trauma center, a plan that could affect where ambulances take some of the city's most seriously injured patients. Tufts treats some trauma cases now, but it is the only major Boston teaching hospital that is not a state-designated adult trauma center. As a result, ambulances carrying trauma patients often bypass that hospital for Beth Israel Deaconess Medical Center, Boston Medical Center, Brigham and Women's Hospital, or Massachusetts General Hospital. Tufts has weathered tough economic times in recent years, and wants to join the system. Opponents, however, say Boston already has too many trauma centers.