An amendment in the Senate version of municipal health care reform would result in the measure coming up short of the $100 million savings target because of its requirement that communities make the same contribution to retirees’ health care as active employees, according to an analysis from a taxpayers watchdog group.

The study from the Massachusetts Taxpayers Foundation identifies at least 50 communities throughout the state that would have their retiree contributions increased, some by hundreds of thousands of dollars in the first year, negating much of the savings gained by limiting employees’ collective bargaining on health care benefits. The report projects that as many as 100 communities may be affected once all are analyzed. Currently, most communities require retirees, whose health costs are higher on average than younger policyholders, to pay higher portions of their premiums than current workers.

The Senate amendment to require equal contributions would undermine reform, the report says, both by eating through some of the savings achieved by redesigning current employees’ coverage and by inflating municipalities’ already unsustainable obligations to retiree health care benefits.

“Municipalities urgently need to address their unaffordable retiree health care liabilities,” the report warns. “The Senate amendment flies in the face of that reality.”

Senator Katherine Clark, D-Melrose, who authored the retiree contributions amendment, said the amendment would protect the most vulnerable while still saving municipalities money. “Everyone involved in this issue has the same goal,” she says, “and that is to achieve savings for municipalities.”

Clark disagrees with the report’s conclusion that the amendment runs counter to municipal reform efforts.

“This is not a zero-sum game – we can do both,” she says. “We can protect seniors, who are on a fixed income, and we can save money.”

Both the House and Senate plans aim to relieve some of the stress of ballooning health care costs on city and town budgets by effectively revoking collective bargaining rights on health benefits and costs. Unions would retain a 30-day negotiating period with city and town managers under both versions, but if no agreement is reached, officials could unilaterally impose their plan.

Both chambers would allow cities to enroll their employees in the state Group Insurance Commission or design their own plan as long as the savings are equal to or greater than the GIC. In the House version, municipal employees would receive 20 percent of any savings in the first year if a community unilaterally imposed a plan or 10 percent for that year if an agreement is reached. The Senate bill gives employees up to one-third of savings regardless of how a new plan is decided.

The Senate proposal also sets up a three-member commission to decide on a plan if union members and city officials can’t agree. It includes a requirement to mitigate the effects on low-income, retired, or very sick employees, and specifically requires that towns contribute the same percentage to retiree and current employee plans.

Employer contributions to employee premiums could still be collectively bargained under either of the reform plans, so yoking retiree benefits to current employee contributions ties communities’ hands for desperately needed future reforms, says the report. Another study by the foundation in February calculated the total non-pension liability to retirees of the top 50 communities in Massachusetts at $20 billion and estimated that the remaining 301 communities and regional school districts would add at least another $5 billion to $10 billion. (Non-pension liabilities consist mostly of health care costs, but also include life insurance and other benefits.)

Geoffrey Beckwith, executive director of the Massachusetts Municipal Association, said that the Senate amendment would “impose significant new cost on communities across the state if they want to use any municipal reform provision that is enacted into law.”

“The contributions of retiree plans have always been outside of the scope of this legislation,” Beckwith pointed out. “We feel it would be not only counter-productive but cost-prohibitive to introduce this new burden now.”

For example, he said, matching retiree contributions to current employee levels would cost the town of Needham $400,000 a year, “which would pretty much wipe out…any savings for taxpayers and negate the reform.” This amount represents an 8.5 percent increase in retirement contributions for fiscal year 2012 or nearly the entire proposed 2012 operating budget of the town’s Health Department.

Other towns that the taxpayers foundation found would be affected include Bellingham and Dedham, which would both see their contributions to retiree plans raised from 50 percent to 80 percent at a cost of more than $260,000 each, and Chelmsford, which would have to absorb an additional $583,000 in the first year alone. Chelmsford’s added cost is the highest of the municipalities that were analyzed, but nine other communities would face increases of more than $250,000 and three of more than $400,000 in the first year.

A joint House and Senate conference committee is working to resolve the differences between the two bills.