WASHINGTON (Reuters) - The U.S. Senate on Wednesday backed an amendment to pension legislation that would give airlines 20 years to fix funding shortfalls in the pension plans they sponsor.

The amendment, by Sen. Johnny Isakson, a Georgia Republican, was passed on a voice vote. It allows struggling airlines with underfunded pension plans to qualify for the relief without freezing their plans, so long as they immediately fund any future pension benefit promises.

The Senate was expected to approve the underlying legislation later on Wednesday. It would require companies sponsoring pensions to set aside more money for retirees over time and pay higher premiums to the deficit-ridden agency that insures the plans, the Pension Benefit Guaranty Corp. (PBGC).

     The White House issued a statement saying the Senate bill was too weak. President George W. Bush's advisers will recommend a veto of the bill that emerges from Congress if its net effect is to weaken rules for pension plans, instead of strengthening them, the administration warned.

     A bill similar to the Senate's has passed two House committees, but it does not have special airline relief.

     Airlines have been lobbying for months for special aid in the pension bill, with Delta and bankrupt Northwest Airlines  leading the effort for assistance.

     "I know that it is critical to some of our airlines that we have this language," Sen. Trent Lott, a Mississippi Republican, said before the vote.

     The amendment would give airlines 20 years to pay off the underfunding in their plans, in addition to the seven years provided by the bill to all companies to fix their pension shortfalls.

     On Wednesday the PBGC warned that its financial health was still in peril even though its deficit eased slightly in fiscal 2005 to $22.8 billion.

     The agency insures defined benefit pensions, which have a fixed payout at retirement, based on salary and years of service. It guarantees the benefits up to certain limits when companies and their pension plans collapse.

     Lawmakers say they want to get pensions on a sounder footing to avoid a possible taxpayer bailout of the PBGC.

     "I do not want another savings-and-loan situation like we had in the late '80s, coming out of bad policy with the PBGC," said Iowa Republican Sen. Charles Grassley, one of the pension bill's co-sponsors.

     "As we've watched this corporation deteriorate rapidly in recent years, the possibility of such a bailout has become increasingly real," Grassley said.

     But the PBGC says its financial problems would actually worsen in the short term under the bills, because they start with short-term relief for companies, and phase in tougher funding rules over several years.