Mr. Schumer said the public option would hold down costs because it would not have to generate profits, answer to shareholders or incur marketing expenses. His proposal would have required the public plan to negotiate rates with doctors and hospitals, rather than setting prices based on Medicare reimbursement rates. Under Mr. Rockefeller’s plan, the payment of doctors and hospitals would have been based on Medicare rates for the first two years.

Mr. Rockefeller said the Congressional Budget Office had estimated that a government insurance plan could slice $50 billion from the cost of Mr. Baucus’s bill, originally put at $774 billion over 10 years. The budget office predicted that eight million people would initially enroll in the public plan  about one-third of those who would seek coverage through new markets, or insurance exchanges.

“The public plan will be optional,” Mr. Rockefeller insisted. “It will be voluntary. It will be affordable to people who are now helpless before their insurance companies.”

But Senator Charles E. Grassley of Iowa, the senior Republican on the committee, said a government insurance plan would have inherent advantages over private insurers. “Government is not a fair competitor,” Mr. Grassley said. “It’s a predator.” He predicted that “a government plan will ultimately force private insurers out of business,” reducing choices for consumers.

Senator John Ensign, Republican of Nevada, said he feared that a government plan would prove so popular it could never be uprooted. “Does anybody believe Congress would let this public plan go away once it has a constituency?” Mr. Ensign asked. “No way. Once it’s started, you will never get rid of it. Congress will subsidize it more and more, allow it to grow and grow.”

Besides Mr. Baucus, two Democrats, Senators Kent Conrad of North Dakota and Blanche Lincoln of Arkansas, voted against both public option proposals. Two other Democrats, Senators Thomas R. Carper of Delaware and Bill Nelson of Florida, voted against the first amendment, but supported the second.

Mr. Carper said he liked Mr. Schumer’s proposal because it “would establish a level playing field” for competition between private insurers and the government plan.