Five times in the past dozen years, bills on medically assisted suicide have risen in the California Assembly, and five times they have failed. In every instance, much of the credit for their demise has gone to a constituency typically associated with advancing personal choice and civil rights — namely, the disability-rights movement.

The latest attempt, Assembly Bill 374, which its backers called the California Compassionate Choices Act, failed to make it out of committee in June. Modeled on a statute passed by Oregon voters in 1997, it would have allowed mentally competent patients whom doctors found had less than six months to live to legally acquire lethal prescription drugs for self-administration.

Many disability-rights activists contend that the increasingly cost-conscious health care system, especially health maintenance organizations, inevitably would respond to legalized suicide by withholding expensive care from the disabled and terminally ill until they chose to end their lives.

“HMOs are denying access to health care and hastening people’s deaths already,” said Paul Longmore, a history professor at San Francisco State University and a pioneer in the historical study of disability. “Our concern is not just how this will affect us. Given the way the U.S. health care system is getting increasingly unjust and even savage, I don’t think this system could be trusted to implement such a system equitably, or confine it to people who are immediately terminally ill.”

Disabled people, Mr. Longmore said, “probably even more than most other citizens, understand the kind of suffering and needless pain that’s inflicted on a lot of people and leaves some of them to prefer to die when they can’t get the help they need.”

The activists believe the success they’ve had in combating assisted-suicide legislation has stemmed in part from their not being branded as religious conservatives and thus having more credibility among liberals and moderates.

“We were not part of a moral or religious overlay, but, rather, we were speaking to the public policy,” said Ann Guerra, who has multiple sclerosis and is executive director of an independent-living center in Grass Valley, Calif.

One of the California bill’s cosponsors, Lloyd Levine, a Democratic assemblyman, said, “We really had two main groups working against us: the Catholic Church and the disability-rights folks.”

Twenty-one states, including some with progressive voting records, have rejected assisted-suicide laws. Not all disability-rights advocates oppose medically assisted suicide. Public opinion polls indicate rank-and-file disabled people tend to favor the practice, given adequate safeguards, but activists have attacked those polls as misleading.

Alan Toy was 3 when polio struck him. The disease cost Toy the use of his legs. At 57, he is an accomplished film actor, a member of the board of directors of the American Civil Liberties Union and founder of Living Independently in Los Angeles, an online resource for disabled people.

Mr. Toy, who spoke out in favor of AB 374, said his fellow disability-rights advocates’ opposition to the assisted-suicide laws “comes out of fear, and I’m tired of living my life from that perspective.”

“If we’re always acting out of the history of all the injustices perpetrated against us, instead of the progress we’ve made, it reinforces societal notions that people with disabilities are victims,” he said. Mr. Toy is convinced the direst fears of opponents of AB 374 are unrealistic.