Senior citizens angry over the AARP's endorsement of the Medicare (search) bill are ripping up or burning their AARP membership cards and flooding the lobbying group's Internet message board with complaints in what could be the biggest revolt in its ranks since the 1980s.

Many fear the Republican-backed bill approved by Congress on Tuesday will harm senior citizens, and they say the AARP (search) — the nation's most influential retiree lobby, with 35 million members — sold them out.

The bill "destroys one of the most successful programs in the history of this country," Isaac Ben Ezra, president of the Massachusetts Senior Action Council (search), said as he led a demonstration of about 40 people here against the bill Monday. "Shame, AARP."

John Rother, policy director at AARP, said Tuesday that the bill was not perfect, but it was a step forward, and the organization will continue to try to improve the law.

"We were either going to get something now or else it wasn't going to happen for many, many years to come," he said.

Rother estimated that, as of Monday, 3,000 to 4,000 AARP members had canceled their memberships.

The law, pushed by President Bush, is the biggest change in Medicare since its creation in 1965, and includes a new prescription drug benefit for 40 million older and disabled Americans. Supporters say it was long overdue; detractors say it was a giveaway to insurers and drug companies.

The law would set up a limited program of competition between traditional Medicare and private plans, beginning in 2010. Activists are worried that could lead to the privatization of Medicare and place the elderly in the hands of "insurance sharks" more concerned about profits than quality medical care. Elderly people have also questioned the AARP's motives, because it has a for-profit arm that earns royalties from the sale of health insurance.

AARP endorsed the plan about a week ago as it headed toward congressional approval. AARP's support was welcomed by Republicans and immediately criticized by the Democrats, who predicted a revolt within the 45-year-old organization.

"It's a firestorm out there. I am absolutely convinced that on this issue AARP doesn't speak for their membership," said Edward Coyle, executive director of the Alliance for Retired Americans (search), which represents more than 3 million retirees.

The dispute could expose a generational rift in the AARP: Many of the angriest protests have come from the elderly — this, at a time when the AARP is aggressively recruiting baby boomers before they reach their golden years.

In West Palm Beach, Fla., Sam Oser, a 77-year-old retiree, organized a protest in his retirement community and burned his AARP card.

"The more we thought about the Republican plan — the more we thought about it, the angrier we got and we felt the AARP was really selling us out," he said.

Julia Kayser, 76, of Easthampton, N.Y., the president of a local AARP chapter, said that during a recent visit to a senior center, where she serves lunch as a volunteer, she told people they ought to quit the AARP.

"A lot of people will not renew their membership when it comes due," said.

Card-burnings and protests were also reported in such places as Washington, D.C., Webster Groves, Mo., and San Francisco.

"We don't think AARP in the least represents seniors on this issue," said Bruce Livingston, executive director of Senior Action Network (search) in the San Francisco area. "We're going to encourage people to quit. This is just the beginning."

Robert Blendon, a professor of health policy and political analysis at Harvard, said he had looked at polls on whether the elderly supported the bill, and predicted there would be discontent among AARP members.

He said it would be difficult to explain to the elderly why it was politically expedient to support the new bill. "They just can't understand why you have to settle for a half a bagel here, with a hole in the middle," he said.

This is not the first time AARP has seen a rebellion among its members.

AARP supported a sweeping Medicare insurance program for catastrophic illness in 1988 despite an outcry from many older Americans, including AARP members, who were angry at having to pay a surtax for mandatory coverage that some did not want or need. The law was repealed in 1989.

At one point, House Ways and Means chairman Rep. Dan Rostenkowski, D-Illinois, was chased down a Chicago street by a group of elderly people opposed to the bill after he refused to discuss it with them.

The AARP was also embarrassed in the early 1990s when it initially seemed to support the Clinton administration's health reform plan, despite a poll in its own magazine that found that members were overwhelmingly against it. AARP ended up applauding, rather than endorsing, the plan.