The Senate Finance version has a decided middle-of-the-road flavor, shunning any provision for the government to sell insurance in competition with private industry. Nor does it require businesses to offer coverage to their workers, although large firms that do not would be required to offset the cost of any government subsidies going to those employees.

The measure would require that millions of Americans purchase private insurance for the first time, and would set up a new marketplace where policies would be available.

Federal subsidies would be available to millions of lower-income individuals and families to help defray the cost of coverage that would otherwise be out of their reach.

The measure would be paid for through a variety of tax increases and spending cuts, including savings of hundreds of billions of dollars from Medicare, the federal health care program for seniors.

Wednesday's report from CBO's director, Dr. Douglas Elmendorf, stressed that the estimates were preliminary.

It said that by 2019, "the number of nonelderly people who are uninsured would be reduced by about 29 million," either through private insurance or by enrolling in federal programs. That would leave an additional 25 million uninsured, about one-third of them illegal immigrants who are not eligible for coverage under the bill.

"Under the proposal, the share of legal nonelderly residents with insurance coverage would rise from about 83 percent currently to about 94 percent," the letter said.