The American Hospital Association, an industry trade group, is starting to lobby against the Medicare for all proposals. Unlike the doctors’ groups, hospitals are not divided. “There is total unanimity,” said Tom Nickels, an executive vice president for the association.

“We agree with their intent to expand coverage to more people,” he said. “We don’t think this is the way to do it. It would have a devastating effect on hospitals and on the system over all.”

Rural hospitals, which have been closing around the country as patient numbers dwindle, would be hit hard, he said, because they lack the financial cushion of larger systems.

Big hospital systems haggle constantly with Medicare over what they are paid, and often battle the government over charges of overbilling. On average, the government program pays hospitals about 87 cents for every dollar of their costs, compared with private insurers that pay $1.45.

Some hospitals make money on Medicare, but most rely on higher private payments to cover their overall costs.

Medicare, which accounts for about 40 percent of hospital costs compared with 33 percent for private insurers, is the biggest source of hospital reimbursements. The majority of hospitals are nonprofit or government-owned.

The profit margins on Medicare are “razor thin,” said Laura Kaiser, the chief executive of SSM Health, a Catholic health system. In some markets, her hospitals lose money providing care under the program.

She says the industry is working to bring costs down. “We’re all uber-responsible and very fixated on managing our costs and not being wasteful,” Ms. Kaiser said.

Over the years, as hospitals have merged, many have raised the prices they charge to private insurers.

“If you’re in a consolidated market, you are a monopolist and are setting the price,” said Mark Miller, a former executive director for the group that advises Congress on Medicare payments. He describes the prices paid by private insurers as “completely unjustified and out of control.”

Many hospitals have invested heavily in amenities like single rooms for patients and sophisticated medical equipment to attract privately insured patients. They are also major employers.

“You would have to have a very different cost structure to survive,” said Melinda Buntin, the chairwoman for health policy at the Vanderbilt University School of Medicine. “Everyone being on Medicare would have a large impact on their bottom line.”

People who have Medicare, mainly those over 65 years old, can enjoy those private rooms or better care because the hospitals believed it was worth making the investments to attract private patients, said Craig Garthwaite, a health economist at the Kellogg School of Management at Northwestern University. If all hospitals were paid the same Medicare rate, the industry “should really collapse down to a similar set of hospitals,” he said.

Whether hospitals would be able to adapt to sharply lower payments is unclear.

“It would force health care systems to go on a very serious diet,” said Stuart Altman, a health policy professor at Brandeis University. “I have no idea what would happen. Nor does anyone else.”

But proponents should not expect to save as much money as they hope if they cut hospital payments. Some hospitals could replace their missing revenue by charging more for the same care or by ordering more billable tests and procedures, said Dr. Stephen Klasko, the chief executive of Jefferson Health. “You’d be amazed,’ he said.

While both the Medicare-for-all bill introduced by Representative Pramila Jayapal, Democrat of Washington, and the Sanders bill call for a government-run insurance program, the Jayapal proposal would replace existing Medicare payments with a whole new system of regional budgets.

“We need to change not just who pays the bill but how we pay the bill,” said Dr. Gaffney, who advised Ms. Jayapal on her proposal.

Hospitals would be able to achieve substantial savings by scaling back administrative costs, the byproduct of a system that deals with multiple insurance carriers, Dr. Gaffney said. Under the Jayapal bill, hospitals would no longer be paid above their costs, and the money for new equipment and other investments would come from a separate pool of money.

But the Sanders bill, which is supported by some Democratic presidential candidates including Senators Kirsten Gillibrand of New York, Cory Booker of New Jersey, Elizabeth Warren of Massachusetts and Kamala Harris of California, does not envision a whole new payment system but an expansion of the existing Medicare program. Payments would largely be based on what Medicare currently pays hospitals.

Some Democrats have also proposed more incremental plans. Some would expand Medicare to cover people over the age of 50, while others wouldn’t do away with private health insurers, including those that now offer Medicare plans.

Even under Medicare for all, lawmakers could decide to pay hospitals a new government rate that equals what they are being paid now from both private and public insurers, said Dr. David Blumenthal, a former Obama official and the president of the Commonwealth Fund.

“It would greatly reduce the opposition,” he said. “The general rule is the more you leave things alone, the easier it is.”