WASHINGTON — Two senators are asking Congress’ investigative arm to find out whether the federal agency that insures private pension plans for millions of Americans has enough money and the right approach to do its job.

Finance Committee Chairman Chuck Grassley, R-Iowa, and the panel’s top-ranking Democrat, Max Baucus, D-Mont., in a statement Tuesday, said that they want the Government Accountability Office to report back by early next year with preliminary information on the Pension Benefit Guaranty Corp., which insures traditional, defined benefit pension plans.

Bankrupt steel and airline companies that have transferred pension responsibilities to the PBGC have been a major factor in the growing demands placed on the agency and its swollen debt.

At the end of 2005, the agency recorded a deficit of $22.8 billion.

In their letter to the GAO, the senators said that 80 percent of all claims against the PBGC have come since 2000.

In 2000, the agency paid benefits of $900 million to 243,000 people. Five years later, PBGC was paying $3.7 billion in benefits to 698,000 people, the senators wrote.

‘‘This rapid increase in participants, benefit payments and investments would be a challenge to any organization,’’ the senators wrote. ‘‘We are particularly concerned in how PBGC is handling this challenge and whether legislative changes are needed in PBGC’s structure, appropriations or law.’’

President Bush in August signed a bill to help the nation’s troubled pension system by shoring up funding for traditional pensions. Supporters hope the changes will prevent a costly taxpayer bailout of the PBGC.

PBGC’s operations are financed by insurance premiums paid by companies that sponsor traditional pension plans. It also earns money from investments and receives funds from pension plans it takes over. The agency is not funded through tax revenues.

The fear, however, has been that a taxpayer-funded bailout could happen at some point if the private pension system weren’t overhauled by Congress.