More than half of SEIU's members work in health care as nurses, doctors, lab techs, or in nursing homes and as home health care providers. Many earn less than $40,000 a year and some have no health insurance coverage.

The union supports the public option. Insuring more Americans could mean coverage for its members and also could increase demand for health care workers, expanding SEIU membership.

Rivera insisted that SEIU's motives aren't business driven, and said that some provisions the union backs could hurt its members. For example, as hospitals increase use of electronic medical records, less staff would be needed to manage files and bills.

"This is a social mission," Rivera said. "If we just could have the peace of mind that nobody's going to lose their home, that they'll have access to health care independent of their economic situation. That's what's important . . . We're not doing this because of the future growth of our union."

AMA: The American Medical Association represents about 250,000 physicians. Members support universal coverage but not necessarily a public option.

The group endorsed overhaul legislation proposed by Democrats in the House of Representatives, in part because it changed a Medicare payment formula that had threatened to cut doctors' payment rates. Doctors also want changes to liability laws.

AMA president-elect Cecil Wilson said the Medicare payment fix, dubbed the "doc fix," is "not just about doctors, it's about access to care for senior citizens. Physicians are increasingly finding they cannot afford to take care of Medicare patients. They're having to limit the number of Medicare patients they see."

As for a public option, Wilson said, "It's not clear what that's going to mean. This is such a loaded term. The longer we focus on this and our efforts on it, the more risk we run of losing a lot of things that are actually more important. Not doing something is not an option."

AARP: The multimillion member organization for people age 50 and up has spent undisclosed sums on ads and lobbying on behalf of expanding health insurance. It's open to but not insistent on a public option.

Spokesman Drew Nannis said a key goal for AARP is expanding affordable coverage for people who are over 50 but too young for Medicare.

AARP and PhRMA are at odds on some key points; for example, AARP wants the government to negotiate lower drug prices through Medicare and to allow drug reimportation.

AARP, meanwhile, partners with insurance companies to offer health care coverage to its members. It has coverage for ages 50-64 as well as various Medicare-related subsidy plans.

Nannis said that AARP would support trimming Medicare overpayments even if that cost its programs. "Our advocacy always drives our private-market solutions," he said.

Health Care for America NOW: This huge umbrella organization, which claims more than 1,000 member groups, supports a public option, and doesn't think that nonprofit co-ops are a satisfactory alternative. Its Web site urges citizen involvement so that "private insurance companies will have to put our health before their profits."

Its steering committee includes a range of liberal-leaning groups, from ACORN to various unions, the NAACP and MoveOn.org. The SEIU is also a member.