Tufts Medical Center nurses are ratcheting up their heated contract dispute with the Boston teaching hospital. Nurses planned to follow a flash mob at South Shore Plaza with picketing from 4 to 6 p.m. Wednesday outside the hospital in Chinatown. Nurses say 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, they say, have transformed the hospital from one of the best staffed in Boston to the worst. For example, according to figures derived from a public database posted by the Massachusetts Hospital Association, each Tufts nurse in pediatrics cares for at least 1.4 critical care patients, compared with 1.03 patients at Massachusetts General Hospital, the nurses union says. To compensate for chronic understaffing, Tufts is forcing nurses to work overtime and to "float" from one area of the hospital to another where they might not be competent, according to the Massachusetts Nurses Association. The group represents 1,200 nurses at the hospital and is trying to prohibit these practices in a new contract.