For more than three months, Mr. Baucus has been negotiating with three Republican senators, and some of their ideas are incorporated in his bill. He predicted that some Republicans and most Democrats would eventually support a version of his bill.

Senator Olympia J. Snowe, Republican of Maine and one of the senators Mr. Baucus has been negotiating with, said she was not ready to endorse the plan, but would continue working with Mr. Baucus with the hope of getting a deal.

The need for action is urgent, Mr. Baucus said, declaring: “Our health care system is simply unsustainable. It’s breaking the bank for everyone, from families to businesses to governments.”

Mr. Baucus’s bill is the least expensive of five major health care bills moving through Congress. The Congressional Budget Office said the expansion of coverage would cost $774 billion over 10 years, compared with price tags of more than $1 trillion for the other measures.

The budget office said the cost of Mr. Baucus’s bill would be fully offset by new taxes and fees, along with savings squeezed from Medicare, so it could reduce the cumulative total of federal budget deficits by $49 billion over the next 10 years. At the end of that period, it said, new revenues and savings would be growing faster than costs, so the bill could also help reduce deficits in the decade after 2019.

Image Senators Kent Conrad, second from left, and Evan Bayh, second from right, discussed what Senator Max Baucus referred to as a good balanced bill. Credit... Stephen Crowley/The New York Times

Like the other bills, Mr. Baucus’s proposal would require most Americans to have health insurance, with financial penalties for those who flout the requirement. But his plan differs from the others in significant ways:

¶Instead of creating a new government health plan, Mr. Baucus would set up nonprofit insurance cooperatives in every state. The Congressional Budget Office said the co-ops “seem unlikely to establish a significant market presence in many areas of the country.” This finding provides ammunition to liberals who say the co-ops could not compete effectively with big insurance companies.