While experts may disagree on the strength of the Pension Benefit Guaranty Corp.’s insurance program for single-employer pension plans, there is a broad consensus that lawmakers had to act last year to prevent the collapse of big, underfunded multiemployer pension plans, and with that the PBGC’s multiemployer insurance program.

Indeed, the likelihood of such a debacle was so obvious that lawmakers passed a multiemployer pension plan rescue package.

The grim numbers were hard to ignore. As recently as 2008, the PBGC’s insurance program that guarantees a portion of benefits promised to the plans’ participants if the plans are unable to do so was in reasonably decent shape with about $1.3 billion in assets and $1.8 billion in liabilities.

But with the looming insolvency of several large multiemployer plans, the deficit in the agency’s multiemployer insurance program ballooned last year to more than $42 billion, many times the roughly $120 million in premiums the PBGC collected from the nation’s 1,400 multiemployer plans, which have about 10.4 million participants.

The U.S. Government Accountability Office warned that the financial drain on the PBGC insurance program from guaranteeing benefits of participants in failed plans would be so great that the insurance program would go broke in the next 10 to 15 years.

To prevent that doomsday scenario from developing, or at least delaying it, lawmakers agreed on a legislative package that would have been inconceivable just a few years ago: allowing trustees of financially troubled plans to cut participants’ benefits. “Lawmakers gave trustees the tools to do this,” said Jack Abraham, a principal with PricewaterhouseCoopers L.L.P. in Chicago.

“Having labor and industry championing a well-crafted proposal that did not require government or taxpayer funding made for the right elements to encourage Congress to act in a timely and in a bipartisan fashion,” said Vince Sandusky, CEO of the Sheet Metal and Air Conditioning Contractors’ National Association in Chantilly, Virginia.

At its core, the new law is basic: Participants’ benefits can be cut if a plan is projected to go insolvent during a current plan year or any of the next 14 years, or any of the next 19 years if the plan’s ratio of inactive participants to active participants exceed 2-to-1 or if the plan is less than 80% funded. Certain participants would be protected from benefit cuts, such as retirees age 80 and older and those receiving disability from the plan. Retirees between the ages of 75 and 79 would face smaller benefits cuts than younger retirees.

For some plans, “very small benefit adjustments” will enable them to survive, Mr. Sandusky said, while others will “need more drastic action to remain solvent.”

Still, he said, “There is that small number that can’t be saved, no matter what.”

That benefit guaranty burden then would be shifted to the PBGC, which, some believe, eventually could overwhelm the agency even with the legislation passed by Congress last year.

The action taken by Congress so far has not drawn cheers in all quarters. Karen Ferguson, director of the Pension Rights Center in Washington, describes it as “shocking” that Congress chose to allow multiemployer plan trustees to cut benefits to current retirees.

A more sensible and fairer approach, she said, would have been to boost premiums multiemployer plans pay the PBGC. While the new law doubled the premium to $26 per participant from $13, Ms. Ferguson says premiums still are “incredibly cheap.”