The Congressional Budget Office said Monday that the Senate health bill could significantly reduce costs for many people who buy health insurance on their own, and that it would not substantially change premiums for the vast numbers of Americans who receive coverage from large employers. The eagerly awaited report, which came as the Senate began debate on the legislation, provided Democrats with ammunition against Republicans who have criticized the bill on the ground that it would raise costs for a majority of Americans. Moderate Democrats whose votes are vital to President Barack Obama's hopes of getting the bill through the Senate, like Sen. Evan Bayh of Indiana, had feared that the bill would drive up costs for people with employer-sponsored coverage. Bayh issued a statement saying the report reassured him on that point. Before taking account of federal subsidies in the form of tax credits that would benefit many people who buy health insurance on their own, the bill would tend to drive up premiums, the budget office said. But including the subsidies, which would cost the government nearly $450 billion in the next 10 years and on average cover nearly two-thirds of the total premium charged to people buying insurance on their own, more people would see their costs decline than would see them rise, the budget office said. For most people who get health insurance through employers -- five-sixths of the total market -- the budget office concluded that there would be very little change in their premiums relative to projections under current law. Administration officials said the report provided a lift to the bill, which embodies Obama's top domestic priority. 'The CBO has rendered a fundamental judgment that this will reduce the deficit and reduce people's premium costs,' said Rahm Emanuel, the White House chief of staff, who met with Senate Democratic leaders on Capitol Hill on Monday. 'All the Republican leadership will guarantee you is the status quo.' But Republican senators like Charles E. Grassley of Iowa and Mitch McConnell of Kentucky, the minority leader, said the report validated their concerns. They focused on the prediction that unsubsidized premiums in the individual insurance market, representing a relatively small share of those who have health insurance, would rise an average of 10-13 percent. 'The analysis by the Congressional Budget Office confirms our worst fears,' Grassley said. 'Millions of people who are expecting lower costs as a result of health reform will end up paying more in the form of higher premiums. For large and small employers that have been struggling for years with skyrocketing health insurance premiums, CBO concludes this bill will do little, if anything, to provide relief.' The Senate majority leader, Harry Reid, Democrat of Nevada, said the highly partisan floor debate that opened Monday afternoon was one of the most significant in the history of the Senate. It is expected to continue for much of December, with supporters and opponents alike offering a raft of amendments as the White House and Democratic leaders seek to cobble together the 60-vote coalition necessary to win passage. Administration officials continued to reach out to lawmakers in both parties to try to build support. Sen. Susan Collins, R-Maine, said she met Monday for 45 minutes with Nancy-Ann DeParle, director of the White House Office of Health Reform, to discuss her concerns about the legislation. In its report, the budget office compared estimates of premiums in 2016 under the new legislation and under current law. In either case, after seven years of inflation, premiums would be substantially higher than they are today. The budget office said the analysis of premiums was extremely complex, so the experience of individuals and families 'could vary significantly from the average.' While it found varying effects on people who buy insurance on their own, it concluded that the overall impact on the individual market would be positive. 'In general,' it said, 'the proposal would tend to increase premiums for people who are young and relatively healthy, and decrease premiums for those who are older and relatively unhealthy.' Under the legislation, it said, the average premium per person in the individual insurance market would be 10 percent to 13 percent higher than under current law. But, it said, most people in this market -- 18 million of the 32 million people buying insurance on their own -- would qualify for federal subsidies, which would reduce their costs well below what they would have to pay under current law. For people receiving subsidies, the budget office said, premiums would be 56 percent to 59 percent lower than under current law. Under current law, annual premiums in that market in 2016 are projected to be $5,500 for individuals and $13,100 for family coverage. Without subsidies, it said, premiums under the bill would average $5,800 a year for individuals and $15,200 a year for families buying coverage on their own. Under current law, those figures would be $5,500 and $13,100. 'This study indicates that, for most Americans, the bill will have a modestly positive impact on their premium costs,' Bayh said. 'For the remainder, more will see their costs go down than up.' Under the bill, the budget office said, individual policies would have to provide more benefits and pay a larger share of costs than most existing policies do. In other words, it said, some people would pay more, but would also get more. Insurers, it said, would have to cover certain services that, in many cases, are not covered by existing policies in the individual insurance market. These include maternity care, prescription drugs, mental health services and substance abuse treatment. Moreover, it said, under the legislation, insurance would cover an average of 72 percent of medical costs for people buying insurance on their own, up from 60 percent today. The budget office said it foresaw 'smaller effects on premiums for employment-based coverage.' In groups with 50 or fewer employees, it said, unsubsidized premiums in 2016 would average $7,800 a year for individuals and $19,200 for families -- scarcely any different from the amounts expected under current law. Of the 25 million people receiving coverage from small businesses, it said, 3 million would qualify for subsidies, which would reduce their premiums by an average of 8 percent to 11 percent. Large employers would generally not be eligible for such assistance. Their premiums in 2016 would average $7,300 for individual and $20,100 for family coverage, the report said. Under current law, the projections are that the comparable figures would be $7,400 for individual coverage and $20,300 for family coverage. 