BOSTON - 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. She also said there's no requirement for hospitals to adjust their staffing levels based on patients' medical needs.

"As a result, hospitals are forcing nurses to ration care, placing patients' health in jeopardy," Kelly-Williams said.

The Massachusetts Hospital Association says the question would mandate one-size-fits-all nurse staffing ratios, and is an attempt by the union to pressure lawmakers into approving a similar bill that has repeatedly failed on Beacon Hill.

"Ongoing care is best determined by the caregiving team at the bedside, not by a government-set fixed formula," said association president and CEO Lynn Nicholas.

Nicholas said applying ratios also runs counter to the state's efforts to contain costs and improve service by rewarding hospitals and other providers based on quality of care delivered and patient satisfaction.

The question would require one nurse for every four patients in medical or surgical units. In emergency departments, the question would require one nurse for up to three patients depending on the severity of the condition. Critical care units would set a maximum of two patients per nurse.

The nurses' association conceded that it's pursuing what it calls "a dual track by keeping the option open to take the issue directly to public via a ballot initiative should the legislature not act."

Both sides also point to California, which adopted a nurse-patient ratio law in 1999 - although each side disagrees about whether that's been a good thing for patients.

The proposed ballot question would also create a standardized formula for rating the illness level of patients and use that measure to determine if the safe nurse-patient ratio should be lowered to ensure more intensive care.

The measure would also require all acute care hospitals to adhere to the new standards. Hospitals found to have violated the law would be subject to a $25,000 fine for each day they fail to be in compliance.

Backers of the question say Massachusetts is one of 10 states along with the District of Columbia to have filed legislation in the past two years to create nurse-patient ratios.