WASHINGTON — The drive to remake the nation's health care system suffered yet another setback in Congress on Thursday when a pivotal group of House Democrats demanded numerous changes in legislation the leadership was drafting on a fast track.

The emerging bill "lacks a number of elements essential to preserving what works and fixing what is broken," 40 members of the Blue Dog Coalition of moderate to conservative Democrats wrote in a letter to party leaders. To win their support, they said, any legislation would need to be much more aggressive in reining in the growth of health care.

The letter addressed to House Speaker Nancy Pelosi and Majority Leader Steny Hoyer also called for greater protections for small businesses and rural health-care providers. It did not specify how much additional time the group wanted, but Rep. Mike Ross, D-Ark., said he believes no vote should take place until September.

That is well past a midsummer informal deadline set by Pelosi, D-Calif.

"I promised the president that we would have legislation out of the House before we went on an August break," Pelosi said earlier in the day. "That is still my goal."

The group issued its letter as Democrats on the House Ways and Means Committee were laboring to put the final pieces in place on a bill that the White House has praised. The party's leadership had hoped to unveil it Friday and push it through committee next week, a timetable that fell apart later in the day. Making the bill public was put off until Monday.

The developments came as a similar timetable appeared in danger of slipping away in the Senate.

There, the Democratic leadership is intent on scuttling a proposed tax on health-care benefits that has long been key to attempts at a bipartisan compromise. At the same time, Senate Majority Leader Harry Reid and others went out of their way to emphasize their interest in gaining Republican support for legislation.

As an alternative, Democrats are considering raising taxes on wealthy investors to help pay for health-care legislation, along with numerous other options, according to officials who spoke on condition of anonymity. The proposal to extend the current 1.45 percent Medicare payroll tax to capital gains earned by high-income taxpayers would bring in an estimated $100 billion over 10 years.

Obama has made health care legislation his top domestic priority, and Democrats in Congress vowed to make it their own, as well, when they returned from their July 4 vacation.

Despite some success — the nation's hospitals agreed to a cut of $155 billion in projected Medicare and Medicaid payments — progress has been scant and internal differences magnified.