That projection led Rep. John Kline, R-Minnesota, who is chairman of the House Education and the Workforce Committee, to comment, "The time to enact responsible reforms is now, before the bomb goes off."

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In a letter sent to every member of Congress, the Pension Rights Center criticized the proposal, saying it would cut retiree benefits from certain multiemployer plans in an "undemocratic" manner. It claimed negotiations are taking place behind closed-doors without public hearings.

The measure, which is supported by such groups as the National Coordinating Committee for Multiemployer Plans, would let trustees cut pensions to 110 percent of the PBGC guarantee level.

But the center called the proposal "ill-conceived," and alleges it "would be devastating to the retirees and widows who count on their pensions to survive."

The proposal would let plans lower benefits – while the plan is still solvent – a move not permitted under the Employee Retirement Income Security Act's anti-cutback rule. That rule says the PBGC can only lower benefits to guarantee levels after assets in a plan are depleted.

In its letter, the center also pointed out that the PBGC believes that most pension plans are adequately funded.

"There is ample time for Congress to develop solutions that are sensible and fair," the center's letter said. It warned against doing anything drastic during the current lame-duck session of Congress.

"Retirees did not cause the plans to be underfunded, and in most cases the plans were well-funded when they left employment," the center said in its letter. "It is unfair to allow multiemployer plans to balance their books on the backs of those who have everything to lose and nothing to gain from this proposal."

As of last month, more than 10 million workers and retirees were covered by 1,510 U.S. multiemployer plans, according to the Pension Rights Center. Between 5 percent and 10 percent of the plans face insolvency, the NCCMP said. That translates to some 1.5 million participants being in plans that will likely fail, the PBGC has said.

The Pension Rights Center's concerns were echoed in another recent letter sent to members of Congress by the International Association of Machinists and Aerospace Workers, which suggests the proposal would lead to "impoverishing" of current retirees.

Instead, the workers' association wants other options reviewed such as raising employer premiums or merging troubled pension plans.

Meanwhile, another coalition, the Partnership for Multiemployer Retirement Security, which says its members include both businesses and some unions, issued its own statement Thursday urging action.

"The longer we wait to take action, the more severe the impact on retirees and workers in the plans in the worst financial shape will become," it said. "The longer we wait, the heavier the burden will become on employers struggling to fund and extend these pension plans."

Its proposal for "deeply troubled" plans calls for "early corrective actions."

As a last resort, these actions would include "partial suspension of accrued benefits for active and inactive vested participants, and the partial suspension of benefits in pay status for retirees. These suspensions would provide, at a minimum, 110 percent of PBGC guaranteed benefits and would be limited to the extent necessary to prevent insolvency," it said.

Proponents of immediate changes to the system claim if Congress does not act soon, the federal government – meaning taxpayers – could end up covering billions of dollars in shortfalls in the troubled multiemployer pension plans.

Brian Newell, a spokesman for the House Education and Workforce Committee, said it was "hopeful Congress will act before the end of the year."