WASHINGTON — Bidding for support from Democrats as well as a single Republican, the chairman of the Senate Finance Committee opened a long-awaited debate over health care Tuesday with fresh plans to reduce costs for working-class families and impose new obligations on the insurance industry.

“This is our opportunity to make history,” said Sen. Max Baucus, D-Mont., who announced he was adding $50 billion to draft legislation to help those who would be required to purchase insurance.

The Republican whom Baucus has courted most assiduously, Sen. Olympia Snowe of Maine, said noncommittally the legislation was “a solid starting point — but we are far from the finish line.”

Democrats hope to push the bill through committee by the end of the week, although it was clear many want to see changes first. In one statement that could cause political heartburn for Democrats and President Barack Obama, the director of the Congressional Budget Office, Douglas Elmendorf, said the proposal could wind up reducing benefits for seniors enrolled in private Medicare plans.

Obama has said, “Nobody is talking about cutting Medicare benefits.”

Elmendorf also estimated in a letter to Baucus that some families with annual incomes in the range of $66,000 could end up spending as much as 20 percent of that — $13,300 — in out-of-pocket health expenses such as premiums and copays.

The panel is the fifth and last of the congressional committees to review Obama’s call for far-reaching legislation to reduce the ranks of the uninsured, expand protections for those already covered and generally reduce the ruinous growth in medical costs nationwide.

Baucus has said he hopes the committee can complete work by week’s end, although more than 500 amendments were pending to the 10-year, nearly $900 billion bill.

Most Republicans were far harsher in their appraisal than Snowe, although they paused in their criticism long enough to praise Baucus for spending months trying to seek a bipartisan compromise.

Sen. Jon Kyl, R-Ariz., called the measure a “stunning assault on our liberty” and cited a requirement for individuals and families to buy insurance.

In a brief statement, Snowe took issue with complaints that health care legislation was taking too long to come to a vote, calling the committee’s work so far “an extensive, meticulous process that places thoughtful deliberation ahead of arbitrary deadlines.”

“Let us recall, it took a year and a half to pass Medicare to cover 20 million seniors, so we simply cannot address one-sixth of our economy, and a matter of such personal and financial significance to every American, on a legislative fast track,” Snowe said.

The changes Baucus unveiled Tuesday seemed designed to appeal to critics within his own party as well as to reach out to Snowe.

The most significant would sweeten the subsidies for individuals and families with incomes up to four times the government’s poverty level — $43,320 for individuals and $88,200 for a family of four. In addition, Baucus called for lower out-of-pocket medical costs for some lower-income families and recommended making it easier for those who cannot afford the coverage offered by employers to qualify for federal subsidies to purchase individual policies.

To hold down costs for older consumers, he also reduced insurance companies’ ability to charge more for coverage on the basis of age, from five times the base rate to four times.

As a result of amendments Snowe and Sen. Charles Schumer, D-N.Y., submitted, Baucus also decided to reduce the penalty for families who defy a proposed mandate to purchase coverage from $3,800 to $1,900.

According to information distributed by the committee, he also incorporated a half-dozen changes backed by Snowe on issues ranging from small businesses to Medicaid.

One of Snowe’s other accepted amendments would be aimed at low-income people who are willing to take on the risk of minimal coverage while still obeying the proposed requirement that all Americans have health insurance; they could sidestep the penalty imposed on the uninsured and pay only a minimal premium. The insurance they could buy, which is essentially catastrophic coverage, is referred to in the bill as the “young invincible” plan.

One highly publicized pet amendment of Snowe’s that Baucus did not accept, however, involved the quasi-public option “trigger” safety net plan. Snowe’s amendment specified that if affordable coverage were not available to 95 percent or more of a state’s residents, a public option would go into effect in that state only.

Kase Wickman of Boston University Washington News Service contributed to this report.