The labor union representing registered nurses at Tufts Medical Center said it has scheduled a Thursday vote to authorize a one-day strike at the Boston teaching hospital as Tufts and union leaders continue to clash over staffing levels. The hospital responded with a statement calling on the union "to resolve its differences at the bargaining table without the threat of a strike." Staffing at Tufts Medical Center and several other hospitals across the state has emerged as an issue in contract negotiations with the Massachusetts Nurses Association, which represents 23,000 members at hospitals, health care clinics, and other sites across the state. The union says that recent cuts in staff and other changes in how they deliver care mean that nurses are caring for more patients at one time on nearly every unit. These changes, its says, have transformed the hospital from one of the best staffed in Boston to the worst. The hospital says that it has shifted some duties previously done by nurses to other staff and that it fares well on various measures of quality care.