WASHINGTON — Longtime service in Congress pays nicely at retirement, with some lawmakers going home with annual pensions of roughly $100,000 — about five times the national average.

So says the National Taxpayers Union, a conservative group that advocates less government spending.

“This is the Rolls-Royce of pension plans,” said David Keating, vice president for the taxpayers’ group, which calculated the congressional pension estimates at the request of The Associated Press. “You can’t get a pension like this in the private sector.”

Reps. Joseph McDade, R-Pa., Lee Hamilton, D-Ind., and Sidney Yates, D-Ill., will be eligible for $98,694 each, about three-fourths of their $133,600 salary, when they leave office in 1999 after a combined 104 years.

Other members’ pensions, which are taxable, could approach $100,000 as well.

According to a 1994 Labor Department study, the latest available, a private sector employee earning $50,000 or more was eligible for pension benefits averaging $17,062.

Why are lawmakers’ retirement benefits so lucrative? A principal reason is that congressional pensions are adjusted each year for inflation, something that sets them apart from most other retirement plans.

Rep. Henry Gonzalez, D-Texas, will be eligible for at least $96,462 if he resigns as planned before the end of his term, for instance. Benefits for Sen. John Glenn, D-Ohio, will exceed $90,000 after factoring in a separate Marine Corps pension for his past duties as a combat pilot and astronaut.

Members of Congress defended the retirement system.

“Almost all my adult life has been spent in public service in the Marine Corps and the Senate,” Glenn said. “I have made the required contributions to the system and will retire from the Senate at the end of next year with a pension based on those years of service.”

Aides for Yates and Gonzalez said the lawmakers deserved healthy pensions after more than three decades of public service. Yates spokeswoman Mary Bain added that the 88-year-old lawmaker could have opted to collect benefits over a longer period by retiring at 65.

Hamilton’s office declined to comment, while McDade’s did not return telephone calls.

McDade, in his 18th two-year term, is the most senior House Republican. Hamilton and Yates have served 17 terms each. Gonzalez is in his 19th term, while Glenn is retiring after four six-year terms.

Lawmakers elected since January 1984 fall under a less generous pension system, but members who joined before then have continued earning pension credits using the old formulas.

In 1995, Congress approved language that would have brought future earnings more in line with those given to other federal workers, but it was part of a broader budget bill vetoed by President Clinton for other reasons. Congress has not passed similar legislation in the current session.

Pension figures aren’t readily available on Capitol Hill, but the National Taxpayers Union has devised a computer program to provide its own estimates based on a survey it took of members of Congress. Members who did not respond were assumed to be in the main civil service pension program.

Members planning to leave Congress after 1998 stand to receive at least $42,000 each if they were elected before 1984, while those under the newer pension system will be eligible for between $12,000 and $29,000 when they hit retirement age.

However, a stock investment plan for the newer members can boost retirement savings by hundreds of thousands of dollars, according to the taxpayers’ group.

Rep. Paul McHale, D-Pa., who joined Congress in 1993 and decided against seeking re-election in 1998, said the days of exorbitant pension benefits are largely over. McHale, 47, will get about $13,000 when he turns 62.

“Pension benefits to newly elected members of Congress are really quite modest, and the vast majority of members [still serving] will fall under the new system,” McHale said.

In general, a member’s pension is based on the number of years of service and the average of the three highest years’ salaries, with a 1991 pay raise helping to boost pension levels.

A pay raise that will take effect in 1998, increasing annual salary to $136,672, will have minimal effect on the pensions, the taxpayers’ group said.

Pension estimates factor in federal service before joining Congress and assume that married lawmakers opt for survivors’ benefits, which lower the annual amount by 10 percent. That option allows a spouse to collect benefits after the death of the lawmaker.