“There is a growing sense that we need to lead on this issue and not wait for it to be offered on the Senate floor,” a senior Democratic aide said. “The idea is that it’s better to show some fight.”

As word of Mr. Reid’s intention spread Thursday, centrist senators from both parties said they had come together in an informal group to resist creation of a uniform nationwide public insurance program.

Leaders of the group, including Senators Ben Nelson, Democrat of Nebraska, and Olympia J. Snowe, Republican of Maine, said they wanted to be sure the bill was not rushed to the floor.

One of the centrists, Senator Mary L. Landrieu, Democrat of Louisiana, said: “I am pressing to get a government-run, taxpayer-supported public option out of the bill. I want to rely on a reformed private marketplace.”

Another member of the centrist group, Senator Susan Collins, Republican of Maine, said: “There are a lot of senators from both sides of the aisle who have a sense of unease about all the bills that have been reported out of committees thus far. In a 60-vote scenario, it is the centrists potentially who will hold the balance of power.”

To get to 60, Mr. Reid would need the vote of Senator Joseph I. Lieberman of Connecticut, an independent who caucuses with the Democrats. Mr. Lieberman, a member of the centrist group, has said he opposes a public option “right now,” but would judge specific proposals in the context of a larger bill.

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The public plan is the most divisive issue in the health care debate, even though the Congressional Budget Office says it would probably attract no more than 12 million people. Critics say the public plan would have unfair advantages and could eventually dominate the market, leaving hardly any role for private insurers.

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Mr. Reid has long said he personally favors a public plan, without saying if it would be in the bill he takes to the Senate floor.

In writing the Senate bill, Mr. Reid is working closely with White House officials and two other Democratic senators, Max Baucus of Montana and Christopher J. Dodd of Connecticut.

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“We have to make a decision pretty quickly,” Mr. Dodd said.

But Ms. Snowe said that in meetings with Mr. Reid she had emphasized “the necessity of taking it slow,” so senators fully understood how the bill would work and how it would affect consumers, businesses and their insurance premiums.

“We must not rush this train out of the station,” Ms. Snowe said. She noted that Democrats suffered a setback on Wednesday when they tried to secure swift approval for a bill increasing Medicare payments to doctors without offsetting any of the cost, $247 billion over 10 years.

“That’s a reflection of what could happen if the Democratic leaders do not smooth the pathway” for major health legislation, by giving senators time to study the details, Ms. Snowe said.

She was the only Republican who voted for a health bill approved last week by the Senate Finance Committee. That bill does not include a government-run plan, though bills approved in July by the Senate health committee and by three House committees do include it.

Ms. Collins, Mr. Nelson and Ms. Snowe said they did not like the idea of a uniform government-run insurance plan even if states could opt out.

Mr. Nelson, a former governor, said, “States should have the opportunity to deal with this without a national public option.” Individual states could set up public insurance programs if they want, he said, or they could band together and establish regional public programs.

In the House, Democratic leaders are so confident of their ability to round up the necessary votes that they have told members of their caucus to be prepared for action on the House floor before Veterans Day, Nov. 11.

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But the House Republican leader, Representative John A. Boehner of Ohio, said he was skeptical. “This talk of the Democrats’ having 218 votes is nonsense,” Mr. Boehner said.

Ms. Pelosi and liberal Democrats favor a “robust” public insurance plan that would use Medicare rates as a basis for paying doctors and hospitals.

That approach received a boost Thursday when 13 House Democrats announced an agreement with Ms. Pelosi that would increase Medicare payments to their states, known for providing high-quality care at a relatively low cost. The states include Iowa, Minnesota, Oregon, Vermont, Washington and Wisconsin.