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When they do, retirees will not be able to rely on PBGC insurance when the insurance program runs out of cash. By law, the multiemployer program provides a fraction of the benefits insured in the single-employer program.

In a recent multiemployer plan insolvency, 40 percent of participants in the plan saw their promised benefits cut by more than half, according to PBGC.

The program's deficit is projected to reach $80 billion by 2026.

A statement from PBGC reflected the severity of the situation:

"The increasing demand for financial assistance from insolvent plans will accelerate the depletion of PBGC's multiemployer program assets. The multiemployer insurance program is in serious trouble and is likely to run out of money by the end of fiscal 2025.  If that happens, the people who rely on PBGC guarantees will receive only a very small percent of current guarantees — most participants would receive less than $2,000 a year and in many cases, much less," the statement said.

Predictions on when the multiemployer program will run out of funds vary and depend on when insolvent plans will need to insured. But under existing law, the program is certain to be depleted by 2036.

Under the White House's proposed budget, PBGC would be able to apply a new variable premium rate on plans, set to the degree to which they are underfunded. That option exists for single employer plans, but Congress has not authorized the agency to apply the more aggressive premium rate structure to collectively bargained plans.

The White House plan would also impose an exit fee on individual companies that leave multiemployer plans, which widens plans' funding gaps by depleting employer and participant contributions.

PBGC says the White House proposal would raise $16 billion over the next 10 years.

The Multiemployer Pension Reform Act of 2014 gave plans in critical and declining status new options to limit benefit payments in order to stave insolvency.

Plans that apply for benefit reductions must satisfy a stringent process overseen by the Treasury Department, and the benefit cuts must be approved by participants.

To date, 15 plans have applied to reduce benefits, but only one plan has been approved by Treasury.

In 2016, the multiemployer program held $2.2 billion in assets. Another $2.8 billion is projected to be collected over the next decade in premium payments currently structured by Congress.

Those assets won't begin to cover the needs of the multiemployer plans in most dire circumstances.

The Central States, Southeast and Southwest Areas Pension Plan is projected to be insolvent by 2025.

In a 2016 letter to participants, the fund's executive director said the retirement benefits of more than 400,000 workers would be reduced to virtually nothing without a government bailout.

During his confirmation hearing, Treasury Secretary Stephen Mnuchin committed to seeking bipartisan resolution to the funding crisis facing the Central States plan.