The Congressional Budget Office said Wednesday that the Alexander-Murray bill would produce a modest reduction in federal budget deficits, but would not substantially change the number of people with coverage.

The budget office estimated that the legislation would reduce deficits by a total of $3.8 billion over the next decade. With the deficit for the last fiscal year alone reaching $666 billion, that is a relatively small number, but supporters of the bill made the most of it.

“The Congressional Budget Office has found that our proposal benefits taxpayers, it benefits consumers — not insurance companies,” said Mr. Alexander, the chairman of the Senate health committee.

The proposal has broad support from Democratic senators and at least a dozen Republican senators. But conservative Republicans in the House and the Senate have denounced it as a bailout for insurers, and Mr. Trump has sent mixed signals about whether he supports it. Representative Dave Brat, Republican of Virginia, called the measure “a nonstarter.”

The budget office said that continuing the cost-sharing payments would not change its estimate of federal spending because it had already assumed that the government would pay the subsidies: $9 billion a year in 2018 and 2019, and a total of $99 billion from 2018 to 2027.

The Trump administration on Wednesday unveiled a new version of the HealthCare.gov website showing premiums and other details of health insurance plans that will be offered for sale in the open enrollment period that starts next week.

In many markets, consumers will have few choices and will see high sticker prices before taking account of financial assistance for which they may qualify. But by carefully shopping around, consumers will often be able to find bargains — a point highlighted in the case in California, where the state runs its own marketplace.