WASHINGTON -- President Barack Obama's main idea for getting quality health care at less cost was in jeopardy Wednesday after key medical providers called his administration's initial blueprint so complex it's unworkable.

Just over a month ago, top officials released long-awaited draft regulations for "accountable care organizations," networks of doctors and hospitals that would collaborate to keep Medicare patients healthier and share in the savings with taxpayers. Obama's health care overhaul law envisioned quickly setting up hundreds of such networks around the county to lead a bottom-up reform of America's bloated health care system.

But in an unusual rebuke, an umbrella group representing premier organizations such as the Mayo Clinic wrote the administration Wednesday saying that more than 90 percent of its members would not participate because the rules as written are so onerous it would be nearly impossible for them to succeed.

"It's not just a simple tweak, it's a significant change that needs to be made," said Donald Fisher, president of the American Medical Group Association, which represents nearly 400 large medical groups around the country providing care for roughly one in three Americans.

The medical groups say they are worried they will be left holding the bag for losses, that the government has designed things so there is no easy way to tell which patients are part of the program, and that there's no reliable way to adjust for patients who are sicker and require closer follow-up and more expensive treatments.

The deadline for public comments on the proposed regulations is still weeks away, but Fisher said "we needed to get their attention early on, so (the administration) could be thinking about how major changes are needed to make these regulations viable."