“A solution that works is going to be challenging for all parties, and that’s going to make it hard to get political buy-in,” said Aliya Wong, the executive director of retirement policy at the U.S. Chamber of Commerce, which has pushed Congress to solve the multiemployer pension problem. “The biggest issue is, where do you get the money from? Every source seems to be tapped out.”

Pension plans across the nation are facing shortfalls, with both corporate plans and those for public employees like teachers and firefighters owing more to retirees than the investment funds can possibly pay. But the looming collapse of the multiemployer pension system is significant given the sheer number of people affected and the potential for a devastating economic ripple effect: retirees losing the pension checks that keep them afloat and a potential wave of bankruptcies among the companies that once employed those workers.

The situation has been brewing since the 2008 financial crisis, as investments plummeted, leaving many plans in the red. The slow economic recovery and recent stock market rally have not been sufficient to reinvigorate the plans, which are jointly funded by labor unions and employers whose workers participate in them.

According to Boston College’s Center for Retirement Research, the nation’s 1,400 multiemployer plans are facing a $553 billion “hole” of unfunded liabilities, meaning they don’t have sufficient assets to cover what they owe workers. About a fourth of these plans are in the so-called “red zone,” where insolvency is more imminent, potentially within the next 10 to 20 years. Most of the participants in these plans work in the transportation, services and manufacturing industries. Their employers, many of which have been trying to withdraw from the plans, include companies like United Parcel Service and Kroger.

U.P.S. said in 2016 that it could be responsible for nearly $4 billion in benefits payments if the Central States Pension Fund, the largest multiemployer plan facing insolvency, slashes benefits to retirees or becomes insolvent. In the past year, U.P.S., which participates in more than two dozen pension plans, has been working with lawmakers on Capitol Hill to help develop pension legislation. It has also offered its own proposals.