House Democrats on Friday answered President Obama’s call for a sweeping overhaul of the health care system by putting forward a 852-page draft bill that would require all Americans to obtain health insurance, force employers to provide benefits or help pay for them, and create a new public insurance program to compete with private insurers — a move that Republicans will bitterly oppose.

The bill was unveiled by a trio of committee chairman, George Miller of Education and Labor, Henry Waxman of Energy and Commerce, and Charles B. Rangel of Ways and Means, who have worked jointly for months to develop a seamless proposal. But the chairmen said they still did not know how much the plan would cost, even as they pledged to pay for it by cutting Medicare spending and imposing new, unspecified taxes.

The three chairmen described their bill as a starting point in a weeks-long legislative endeavor that they said would dominate Congress for the summer and ultimately involve the full panorama of stakeholders in the health care industry, which accounts for about one-sixth of the nation’s economy. They described their efforts as the historic culmination of a half-century of failed attempts across the tenure of a dozen presidents.

Mr. Miller, a Democrat of California, said that completing a bill would require extraordinary cooperation among lawmakers. “In order to change American’s health care system,” he declared, “Congress itself must change.”

House Republicans, who have had no involvement in the development of the health legislation so far, quickly denounced the Democrats’ proposal as a thinly disguised plan for an eventual government takeover of the health care system.

“Families and small businesses who are already footing the bill for Washington’s reckless spending binge will not support it,” the Republican leader, John A. Boehner of Ohio, said in a statement. “Raising taxes, rationing care, and empowering government bureaucrats — not patients and doctors — to make key medical decisions is not reform.”

The House Democrats’ plan is one of three distinct efforts underway on Capitol Hill to draft the health overhaul legislation. In the Senate, both the Finance Committee and the health committee have separate bills in the works, and in recent days those efforts seem to have stumbled.

The health committee, led by Senator Christopher J. Dodd, Democrat of Connecticut, in the absence of the panel’s Democratic chairman, Edward M. Kennedy of Massachusetts, began its public drafting sessions earlier this week. But the process was hampered by a much higher-than-expected cost projection from the Congressional Budget Office, which gave Republican critics ready artillery to fire at the bill.

In the Finance Committee, the chairman, Max Baucus, Democrat of Montana, announced that he had postponed his drafting process, expected to begin on Tuesday, until the week after July Fourth, to give him time to try to reduce the cost of his measure. The Finance Committee bill, being developed jointly with Senator Charles E. Grassley of Iowa, the senior Republican on the panel, may have the best chance of winning bipartisan support.

The House proposal unveiled on Friday was a decidedly progressive measure, which reflected many of the ideas championed by the White House, including such initiatives as the creation of public insurance plan, which Republicans have said they will never support.

In the Senate, lawmakers have been working on a number of potential compromise proposals, including the creation of nonprofit health care cooperatives that could compete with private insurers but would be regulated rather than controlled by the federal government.

The House proposal also included a requirement that employers either provide health insurance or pay a fee equal to 8 percent of their payroll. The House chairmen said that the 8 percent figure, along with virtually every other aspect of the draft legislation, was negotiable and intended as a starting point for deliberations.

But Republicans have voiced opposition to imposing any such requirement on employers, arguing that it would effectively lead to the elimination of jobs.