The two leaders of the Senate committee responsible for most corporate and tax legislation presented a major proposal for strengthening the nation's pension system yesterday in the hope of avoiding further catastrophic failures similar to what happened at United Airlines this year.

The bipartisan proposal, released yesterday by Charles E. Grassley, Republican of Iowa and chairman of the Senate Finance Committee, and Max Baucus of Montana, the ranking Democrat on the committee, expands on a bill the two senators introduced in January, before United defaulted and left the federal pension guarantor to pay more than $6 billion in unfunded benefits.

Senator Grassley said the revised bill was "meant to prevent a real crisis and bring pension rules into the 21st century." Many analysts have warned that the pension failure at United encourages other major airlines to jettison their pension plans to remain competitive, throwing the burden of paying tens of thousands of employees onto the struggling federal pension insurance agency.

There are also concerns that the failure at United is a harbinger of pension problems brewing in other sectors besides the airlines.

In general, the Senate bill appears more closely in line with changes the Bush administration has been urging than does its counterpart bill in the House of Representatives. The House bill, introduced in June by Representative John A. Boehner, a Republican of Ohio, acknowledges that the pension system needs to be strengthened, but proposes changes that are less likely to be objectionable to business.

Companies that offer pensions have warned repeatedly that if Congress enacts rules that are too demanding, they might stop offering pensions at all.

But Senator Grassley warned, in a statement, that to accept that argument says "that the status quo is O.K. and that we should all bury our heads in the sand."

The new Senate measure includes a number of provisions that echo goals laid out by Elaine L. Chao, the labor secretary, in a speech last January. Ms. Chao said that the pension rules had become hopelessly complex and needed to be made both simpler and tougher.

One crucial element of the new Senate bill would end the practice of "smoothing" pension numbers, something companies now do routinely to avoid big year-to-year swings in the contributions they make to their pension plans. Critics of smoothing say it can camouflage severe insolvencies, allowing companies to shrink or skip pension contributions entirely in years when their plans desperately need the money.

Companies and their actuaries have argued that smoothing is valid and essential to running pension plans affordably. They also say that unsmoothed, or "mark to market" pension numbers, can show a great deal of irrelevant volatility and would unduly alarm employees who saw them.

An aide to Senator Grassley said that the bill addressed concerns about affordability, by limiting the amount any company's pension contribution could increase from one year to the next. But he said the limit would affect only the amount contributed, not the calculations leading up to the contribution. He said the senators found unpersuasive the argument that unsmoothed pension values would needlessly frighten employees.

A second major feature of the bill would be to make companies measure the pensions they have promised employees in a way that took their employees' ages into account. That would, in turn, force companies to set aside more money as their workers come closer to claiming their benefits.

But companies have consistently opposed the age-based approach, making it one of the most contentious elements of the pension-overhaul debate. The House pension bill has a compromise measure, requiring companies to account for workers' ages only to an extent.

The Senate bill also includes a novel measure suggested by Ms. Chao in January, the use of a tough new standard for pension funds run by companies whose credit ratings fall to junk-bond levels. The administration proposed such an "at risk" standard because a pension fund's prospects are determined, to some degree, by the health of the company that sponsors it. Many pension funds weakened sharply during the last bear market, but stronger companies were able to revive their pension funds by making big contributions.

The full Senate Finance Committee is scheduled to vote on the new measure next Tuesday.