BOSTON (AP) — A union representing Massachusetts nurses is pushing a ballot initiative designed to set limits on the number of patients that can be assigned to a nurse at any one time. The question would also require hospitals to adjust nurses' patients assignments based on the severity of the medical needs of those in their care, the Massachusetts Nurses Association says. Union officials say patient limits can reduce the number of mistakes, serious complications and preventable readmissions. Massachusetts has no law that established the maximum number of patients a nurse can safely care for at any one time, said Donna Kelly-Williams, president of the Massachusetts Nurses Association/National Nurses United.