As Senate Democrats show tentative signs of coalescing around a compromise health care bill that eliminates the controversial public insurance option, some of their allies are wasting no time in trying to sink the new proposal.

While lawmakers await a Congressional Budget Office cost analysis of the proposed substitute, doctor and hospital groups are already trying to scuttle part of the plan that would allow people ages 55 to 64 to enroll in Medicare. They claim that the provision would overburden the government-run health insurance plan for senior citizens, and they say Medicare already shortchanges doctors and medical providers in its current form.

Liberal groups, including MoveOn.org and union-backed Health Care for America Now, have also railed against the compromise proposal because they view the public option as central to any health care reform effort.

Hospital trade groups argue they have made enough concessions after agreeing last summer to $155 billion in Medicare and Medicaid cuts over a decade.

“Such a policy will further negatively impact hospitals — after we have already agreed to contribute the maximum level of sustainable reductions that community hospitals can reasonably endure,— warned an e-mail alert sent by the Federation of American Hospitals to its members last week after news of the proposed deal first surfaced. The e-mail asked federation members to encourage Democratic Senators to oppose the measure.

“Don’t wait! Click here to take action against a Medicare buy-in,— it said.

The American Hospital Association issued a similar alert to its members, saying, “Adding millions of people to these programs at a time when they already severely underfund hospitals is unwise and should be opposed.—

The president of the American Medical Association also weighed in on the proposed Medicare buy-in. Writing in his blog posted on the AMA Web site, J. James Rohack, a Texas physician, panned the idea, saying, “It would add millions of more patients to a program where it is difficult for a new enrollee to get an appointment with a physician.—

AMA spokeswoman Lisa Lucas said Rohack’s blog writings reflected the official views of the doctors’ group.

“I can almost guarantee we will be out publicly opposing it,— she said.

These groups, however, have refrained from criticizing the other element of the compromise — a measure that would authorize the Office of Personnel Management to negotiate with nonprofit health care plans to offer coverage for individuals who cannot get insurance. The OPM already oversees the Federal Employees Health Benefits Plan.

On his blog, Rohack wrote that a better way to cover the uninsured would be to allow individuals to buy into the federal employees’ plan.

The AMA has been more supportive of the Democrats’ health care plan than other doctors’ coalitions and business groups. The group backed the House health care bill. The AMA also recently wrote a letter to Senate Majority Leader Harry Reid (D-Nev.) that supported some provisions of the Senate plan while expressing reservations about others.

Not everyone has come out swinging against the Medicare buy-in. One major player in the health care debate, the drug companies, has been notably silent.

A spokesman for the Pharmaceutical Research and Manufacturers of America said the group wanted to see a fleshed-out version of the proposal before taking a stand.

“The devil is in the details, and we haven’t seen any details yet,— PhRMA Senior Vice President Ken Johnson said.

The drug companies, which agreed to $80 billion in concessions over 10 years, have generally been supportive of health care reform efforts. Johnson also said the drug companies “have supported initiatives to expand access to coverage.—

The pharmaceutical industry has focused its opposition mostly on efforts to allow importation of drugs from other countries — which proponents say would bring down costs but which Johnson said would lead to unsafe drugs on the market.

The health insurance industry, led by its trade group, America’s Health Insurance Plans, has been highly critical of the legislation, including the public option.

AHIP has been equally critical of the Medicare buy-in but has not taken a position on allowing people to enroll in nonprofit plans overseen by the federal government.

Despite the critical reception that the proposed compromise has received from many of these groups, lobbyists did not say whether they would work to defeat legislation if the plan was ultimately accepted by the Senate.

Robert Blendon, a health policy expert at Harvard University, said the doctor and hospital groups may grow more receptive to the Medicare buy-in if the Senate improves the reimbursement rates for the government-run seniors’ health insurance program.

“They are very concerned about the payment issue,— he said.

On the other hand, Blendon said the progressive groups, such as MoveOn, are more likely to continue to oppose the compromise because they are ideologically committed to having a public option in the overall plan.

“The liberal organizations will keep pounding away,— he said.

By the same token, he said conservative groups, which have opposed the health care plan from the outset, are not likely to be pacified by having the public option removed.

A number of conservative groups are sponsoring a rally against the health plan at the Capitol on Tuesday.

Furthermore, a business coalition that includes the U.S. Chamber of Commerce, the National Association of Manufacturers and the National Retail Federation, said it was releasing two new ads opposing the Senate health care bill. The ads will air on national cable and in nine states represented by moderate Democrats, including Alaska, Arkansas, Connecticut, Louisiana, Nebraska and Virginia.