WASHINGTON -- Comedians love to needle AARP as the unflagging advance team for senility. They're the folks who send the dreaded membership invite when you hit 50, a reminder that you're well on your way to that last roundup.

David Letterman, for one, suggested that Sylvester Stallone's latest geriatric version of Rambo should qualify for "an AARP discount on ammo."

Life as a punch line can present certain annoying challenges, much like the achy joints and ever-failing vision that usually accompany aging. But look who's laughing all the way to the early-bird special.

Fifty years after its founding by a retired high school principal, AARP is a premier lobbying power in Washington. Its 40 million members, many more than capable of finding an Election Day voting booth, make it the country's single largest organization -- that is, if you don't count the Catholic Church.

As it began its three-day anniversary party in Washington yesterday, AARP wasn't renting out a bingo hall for its opening ceremony. Try the steps of the Lincoln Memorial, where stars of the over-50 set -- actress Sally Field, moonwalker Buzz Aldrin and NASCAR speedster Richard Petty, among them -- were offering testimonials. At the Washington Convention Center over the next several nights, performances by Chaka Khan, Chicago and Paul Simon are apt to inspire -- if not a love-in -- then at least a massive outbreak of air guitar.

The celebration's geography is a reminder of AARP's muscle in Washington, its home base, where five years ago it helped President Bush persuade Congress to add prescription drug benefits to Medicare. Two years later, AARP flexed again, leading opposition to private Social Security accounts.

"It's the largest, most-effective lobbying group in D.C., and probably the world," said American University government professor James Thurber, who studies lobbying. "It can stimulate people out there very quickly and get them to push back on issues. Just the threat of AARP against you gets people to change their minds."

Most recently, AARP has mounted a "Divided We Fail" campaign, urging political leaders to set aside partisan rancor to reform health care and strengthen long-term financial security. The campaign's logo is the silhouetted fusion of the Democratic donkey and the Republican elephant.

If the message seems a bit utopian for Washington, it serves another purpose: spreading AARP's name. As much as anything, AARP is a brand -- created in 1999, when the group performed a nip-and-tuck on its former title, the American Association of Retired Persons.

By dropping the "Retired" and adopting a four-letter name, AARP made itself more palatable to the baby boomers it was counting on to help churn hundreds of millions a year in revenue through membership fees and services. There are AARP television shows, a newspaper and glossy magazines that cater to those boomers using cover stories about forever-young types like Caroline Kennedy (50!) and Jack Nicholson (71!). "Secrets of Your SEX DRIVE," began a recent headline. "Why You Want It, When You Want It ... And How to Want It More."

"AARP is a giant with two heads," said Ken Dychtwald, an author and gerontologist. "On the one hand, it has got strong currents of altruism and moral high ground. On the other, it's a rip-roaring marketing engine.

"If you're trying to grow revenue and gain new members who are turning 50, then what they're doing is brilliant. However, if your proposition is to be the defender and the advocate of the poor and elderly, it could be argued that you're a bit turning your back on them."

Bridgeville, Pa., native William Novelli, AARP's avuncular chief executive, waves off the criticism with the easy laugh of an elder dismissing a whippersnapper. "Those revenues are what makes it possible to speak on behalf of older Americans," he said. "Size and success brings criticism. I have an uncle who was a bombardier. He used to say, 'If you're not taking flak, you're not over the target.' "

Mr. Novelli, co-founder of the Porter Novelli public relations firm specializing in health and social issues, joined AARP in 2000 and become chief executive the next year. His mission: To lasso millions of baby boomers old enough to join. "The question was," he said, "could AARP appeal to the boomers and stay relevant?"

He took over a sprawling nonprofit group, now with a $1.3 billion budget and 2,400 employees. It generates more than $1 billion in revenue, a stream produced by $12.50-a-year membership fees, grants and investments, royalties from endorsing everything from health insurance to motorcycle insurance, and by selling advertising space in its publications ($483,000 for a full-page ad in the magazine).