Sens. Edward M. Kennedy, D-Mass., and Christopher Dodd, D-Conn., said in a letter to colleagues that their revised plan would cost dramatically less than an earlier, incomplete proposal, and help show the way toward coverage for 97 percent of all Americans.

In a conference call with reporters, Dodd said the revised bill had brought "historic reform of health care" closer. He said the bill's public option will bring coverage and benefit decisions driven "not by what generates the biggest profits, but by what works best for American families."

The Congressional Budget Office, in an analysis released Thursday evening, put the net cost of the proposal at $597 billion over 10 years, down from $1 trillion two weeks ago. Coverage expansions worth $645 billion would be partly offset by savings of $48 billion, the estimate said.

However, the total cost of legislation will rise considerably once provisions are added to subsidize health insurance for the poor through Medicaid. Those additions, needed to ensure coverage for nearly all U.S. residents, are being handled by a separate panel, the Senate Finance Committee. Bipartisan talks on the Finance panel aim to hold the overall price tag to $1 trillion.

The Health Committee could complete its portion of the bill as soon as next week, and the presence of a government health insurance option virtually assures a party-line vote.

In the Senate, the Finance Committee version of the bill is unlikely to include a government-run insurance option. Bipartisan negotiations are centered on a proposal for a nonprofit insurance cooperative as a competitor to private companies.

Three committees are collaborating in the House on legislation expected to come to a vote by the end of July. That measure is certain to include a government-run insurance option.

At their heart, all the bills would require insurance companies to sell coverage to any applicant, without charging higher premiums for pre-existing medical conditions. The poor and some middle-class families would qualify for government subsidies to help with the cost of coverage. The government's costs would be covered by a combination of higher taxes and cuts in projected Medicare and Medicaid spending.