Trustees of the Central States, Southeast and Southwest Areas Pension Fund are urging a group of federal lawmakers to reassess their opposition to the massively underfunded multiemployer pension plan's move to slash participants' benefits.

Under that proposal, the Central States plan, which at the end of 2014 had $35 billion in liabilities and just $17.8 billion in assets, would cut benefits earned by 272,600 of the plan's more than 400,000 participants.

Such cuts are allowed, if approved by U.S. Treasury Department regulators, under a 2014 federal law intended to prevent the collapse of financially troubled plans and a Pension Benefit Guaranty Corp. insurance program — now in the hole by more than $50 billion — that guarantees a portion of participants' promised but unfunded benefits.

In letters sent earlier this month to Kenneth Feinberg, a veteran attorney and arbitrator whom the Treasury Department named last year to review plan applications to cut benefits, dozens of congressmen, including presidential candidate Sen. Bernie Sanders, I-Vt., urged regulators to reject the cuts.

Those cuts, which could be as large as 50% to 70% of participants' promised benefits, “will bring severe harm to Central States retirees and set a dangerous precedent for other pension plans around the nation,” the senators' letter warned.

“We do not believe that placing an inordinate burden on middle-class workers and retirees is the only option for Central States. For these reasons and more, we urge the Treasury Department to deny this application and move forward with a more equitable plan,” according to the letter signed by 84 members of the House of Representatives.

But in a letter sent last week to the lawmakers, Central States trustees' chairmen Arthur H. Bunte Jr. and Charles Whobrey warned of the devastating consequences if participants' benefits are not cut.

The Central States plan, whose annual benefit payouts now exceed contributions by more than $2 billion, will be unable to pay benefits in about 10 years, the letter noted.

The plan's unfunded benefit liability is so large that without cutting benefits, both the plan and the PBGC's multiemployer pension plan insurance program will go broke, according to the trustees' letter.

“This means that the more than 400,000 hardworking Americans covered by Central States face the following stark and tragic reality: if and when both Central States and the PBGC become insolvent, their benefits will be reduced to almost zero,” the trustees wrote.

If lawmakers are serious about wanting to protect plan participants, they have only two courses of action, according to Thomas Nyhan, the executive director of the Roseland, Illinois-based Central States plan.

“If members are serious about helping these retirees and beneficiaries, they need to either support our rescue plan under the legislation that Congress already enacted, or enact new legislation that provides additional funding. The hard truth is no other course of action will protect participant benefits,” Mr. Nyhan said in a statement Wednesday.

The Treasury Department has until May 7 to act on the Central States plan to cut benefits.