ONE OF THE THINGS that Congress neglected to do before it left town last month was rescue the nation's pension insurance system. That safety net was erected in 1974, when Congress transformed the law so employers no longer could walk away so easily from promises to pay retirement income to their employees. Companies with pension plans were assessed a fee, so much per covered worker per year. The Pension Benefit Guaranty Corp. used the fees to help pay benefits when companies went under, or when plans were terminated for other reasons without enough money to cover their costs.

For some time the system worked; since its creation PBGC has taken over nearly 1,100 plans. But more recently the corporation's obligations have begun to outstrip its foreseeable income. The law leaves PBGC se: for the sake of the workers involved, it has to take over a plan a company abandons. Some companies, including some very large ones, have taken advantage of this to abandon plans they possibly could have sustained -- in a sense shifting a burden to their competitors.

A year ago PBGC's obligations were already about $400 million greater than its likely future ability to pay. Then last summer Allis-Chalmers Corp. dropped about $160 million more in obligations on the agency. Last fall the Wheeling-Pitt Steel Corp., from bankruptcy court, cast off an estimated $425 million more. Other troubled steel companies are also eyeing the agency as a possible source of relief.

Congress moved to deal with the problem last fall in the so-called reconciliation bill required by its budget resolution. The bill was meant to make structural changes in a broad array of programs to reduce their cost and the deficit. The PBGC provisions would have 1) made it harder for companies to slough plans off and 2) raised the fee for solvent companies from $2.60 per employee per year to $8.50. But the reconciliation bill failed in the final hours of the session because the House and Senate could not agree on an unrelated proposal to finance the expansion of the Superfund.

The failure of the reconciliation bill was an embarrassment to Congress on fiscal grounds. In several areas it was costly on policy grounds as well. The members may not have the heart to try again on reconciliation when they return. But in that case the least they can do is pass a pension bill. And quickly.