“If Democrats push a health bill through the Senate using budget reconciliation procedures, the bill would lack Republican support and would lack the support of key constituencies  certainly the business community,” said E. Neil Trautwein, a vice president of the National Retail Federation, a trade group. “Health care reform would crater for this year.”

House Democrats say the Republican protests are overheated. The fast-track procedures have been used 19 times since 1980 to pass major legislation, including much of President Ronald Reagan’s domestic policy agenda in 1981, welfare overhaul in 1996 and President George W. Bush’s tax cuts in 2001 and 2003.

The committee chairmen writing the Senate health bill, Max Baucus of Montana and Edward M. Kennedy of Massachusetts, both Democrats, have been assiduously courting business groups and labor unions, consumer groups, doctors, hospital executives and other health care providers.

Those groups  eager for a seat at the table, eager to sound constructive  have been remarkably restrained so far. They have held back in their criticism of proposals being seriously considered by Congress and the White House. But the strains are beginning to show. Labor leaders have conveyed their concern about taxing health benefits to Mr. Baucus in the strongest possible terms. Employers have warned Congress against requiring them to provide any specific amount of insurance.

Steven Kreisberg, director of collective bargaining and health care policy at the American Federation of State, County and Municipal Employees, said: “We absolutely oppose a change in the tax treatment of employee health benefits. It would endanger the current employer-based health care system at a time when we are trying to sustain it.”

Helen Darling, president of the National Business Group on Health, said, “Most of our members are big employers, and they offer health benefits, but they do not like an employer mandate.” If the government specifies a minimum package of benefits, Ms. Darling said, it could quickly become more comprehensive, without any significant cost controls.

Many lawmakers have promised to give all Americans access to the same health insurance they have as members of Congress. But the package of benefits available to Congress and other federal employees is more generous and more costly than what many private employers offer.