But CBO's estimated deficit is $101 billion, according to its new report.

That $48 billion difference can be explained by several factors. CBO's 20-year estimate starts and ends a year later than PBGC's, which raises the estimate.

The extra year accounts for altered economic projections made since 2015, when PBGC last published it Projection Report.

And PBGC does not include an adjustment for market risk in its estimate, but instead relies on discount rates based on U.S. Treasuries, according to PBGC's report.

The absence of a market-risk adjustment is the greatest factor between CBO and PBGC's estimates of the multiemployer program's deficit, CBO says. If CBO did not account for market-risk, its estimate of PBGC's deficit would be $58 billion, which is $43 billion less than the $101 billion deficit when accounting for market risk.

Dramatic premium shortfalls

Under existing law, CBO expects the multi-employer program to receive $9 billion in claims between 2017 and 2026, and collect $4 billion in premiums in that time. The program's estimated $2 billion in assets will be exhausted by 2025, meaning $3 billion in claims for retirees expected to be made between 2025 and 2026 would not be paid.

Claims for financial assistance are expected to be even greater in the following decade, CBO says.

Between 2027 and 2036, after its assets are projected to be exhausted, CBO expects $35 billion in claims from insolvent multiemployer pension plans. PBGC is only expected to receive $5 billion in premiums during that period.

Evaluation of proposed remedies

In its report, CBO examined several policy proposals that "could" improve the financial health of critically underfunded multiemployer plans, and potentially reduce claims to PBGC in the short and long term.

Options included changing the terms of the insurance PBGC offers participants in insolvent plans, measures to improve plan funding so that some underfunded plans can be put on a healthier course and avoid future insolvency, and potential federal "financial assistance" to PBGC, according to CBO's report.

Providing a bailout to PBGC would limit its risk from plans facing insolvency in the near term, the report says.

Under one option, Congress could give PBGC $10 billion to partition assets in the worst-funded plans. That option, which would include dramatic benefit reductions for participants in those plans, would be designed to get the plans on course to an 80-percent funded ratio, which means they would avoid complete insolvency down the road.

In order to fund the $10 billion bailout to partition the most troubled plans, premiums on all multiemployer plans would have to more than triple, according to CBO.

Another bailout option would be for Congress to guarantee the PBGC. Under current law, the government does not guarantee PBGC's insurance programs.

CBO says $34 billion in federal assistance would be needed to cover all obligations through 2036. If federal assistance were combined with higher contributions from sponsors and participants in underfunded plans, then $26 billion in federal funding would be needed to guarantee the multiemployer insurance program.

Privatization

A third option would be to privatize the multiemployer insurance program, and let insurers in the private sector compete to underwrite policies for different plans.

That option would first require Congress to recapitalize PBGC before the private sector would be willing to assume the large obligations.

Under a privatized solution, CBO expects insurers would have to set aside $101 billion in assets to cover expected obligations. In exchange, participants' premiums would have to go up dramatically, to $286 per participant in a plan that is 80 percent funded. That would be a nearly 11-fold increase from per participant premiums in 2016.

Privatization comes with risk, as large market shocks could bankrupt insurers, as was the situation in 2008, in which case Congress would have to consider bailing them out.

Privatization could also encourage multiemployer plan sponsors to shift to defined contribution plans, as private sector insurance premiums, which CBO says include marketing and overhead costs not included in PBGC's premiums, could be too much for sponsors and participants to bear.

An alternative could be partial privatization, says CBO, where the government guaranteed private insurers against catastrophic losses.

Or the feds could offer insurance alongside private insurers, limiting the government's role in stable economic conditions, and scaling up during a crisis to ensure continued insurance protections.

Partial privatization would reduce some of the risks in full privatization, but the government would still bear some risk, says CBO.

"Lawmakers would need to strike a balance between the continued availability of reasonably priced insurance and the cost of subsidies inherent in the government's role in providing insurance against catastrophic losses," the report says.

A full copy of CBO's report can be accessed here.