The debate over ballot question one has grown contentious, and now the independent state agency tasked with monitoring Massachusetts' healthcare spending is weighing in.

If passed, the proposed law would mandate strict nurse staffing ratios, limiting how many patients one nurse could care for in hospitals across the state.

The pro and con campaigns have offered wildly different cost estimates. Those advocating for the ballot measure say this law would cost about $47 million a year. That’s a fraction of what the opponents estimate. They suggest the law could cost $1.3 billion the first year and $900 million a year after that.

The Massachusetts Health Policy Commission has conducted its own study, which was released today. The agency estimates that if ballot question one passes, it could cost between $676 million and $949 million annually.

The agency says this estimate comes from the cost of hiring a few thousand more nurses — which is necessary to comply with the law — predicted wage increases, and the cost of implementing the law at state-operated hospitals. And, the analysis concludes, this is likely a conservative estimate because of limited available staffing data.

The Health Policy Commission warned that this cost could hurt hospitals’ margins and could lead to a closure in unprofitable units and a reduction in hospitals’ non-healthcare workforce. The agency also suggested it is possible that the cost could be passed on to consumers via higher insurance premiums.

The agency said it found that the measure could lead to some cost savings as well, as it predicts the law would lead to shorter patient hospital stays and less turnover among nurses in hospitals. The HPC estimates that these effects could save the state $34 to $47 million. The Health Policy Commission also looked at the law’s potential impact on patient outcomes and quality of care. For this, they hired Joanne Spetz, a professor at the University of California San Francisco's Institute for Health Policy Studies, who has studied this issue.

Spetz’s analysis focused on data from California, where a similar law was implemented. She found that, overall, patient outcomes in California did not improve as a result of the law.

“Some outcomes got worse, some got better,” says Spetz. “It’s very hard to conclude that the California regulations systematically improved quality of care across all outcomes.”

Several members of the Health Policy Commission also warned that there are differences between the Massachusetts and California laws and that the Massachusetts law could be more challenging when it comes to implementation.

They pointed out that the California law was rolled out over five years, during which time the healthcare system could train more nurses and hospitals could plan for how to comply with staffing ratios. In Massachusetts, the law would go into effect in Jan. 2019 — less than two months after voters decide on whether to make the measure law.

“It seems to me it would be totally impossible for hospital to meet [the timeline],” says Rick Lord, who serves on the commission but did not spearhead the analysis.

Stuart Altman, the chair of the commission, pointed out the Massachusetts law less flexible than the California law.

The commission’s analysis largely aligns with the arguments advanced by those opposing the ballot question. The advocates for the measure — the Massachusetts Nurses Association, a nurse union — has questioned why the agency looked into this measure and whether there was bias.

“Right now, [The Health Policy Commission is] using the hospital data, and we were never part of the process whatsoever,” says Julie Pinkham, executive director of the Massachusetts Nurses Association. She says she was only informed about the agency’s analysis two weeks ago.

Traditionally, Pinkham adds, the Health Policy Commission doesn’t get into analyzing laws. “So, they are breaking new ground,” she says.

Countering the advocates’ accusations of bias, those opposing the ballot measure point out that the Massachusetts Nurses Association trusted the Health Policy Commission enough to write them into the law.

“They wrote the Health Policy Commission nine times into their law as an arbiter, as a third-party validator, to put the regulations in place,” says Dan Cence, who is with the campaign that opposes the measure.

Cence says the report confirmed his concerns. “This is a terrible day for the healthcare system in Massachusetts, and should this ballot measure move forward, it would be a horrible day.”

The commission will further discuss the study's findings at a two-day hearing Oct. 16 and 17.

Read more about ballot question one.

Correction: An earlier version of this story defined 'non-healthcare workforce' differently than the proposed law.