The 61-to-39 vote on health benefits for women would, in effect, override new recommendations from a federal advisory panel that said routine mammograms should begin at age 50, rather than 40.
Senator Barbara A. Mikulski, Democrat of Maryland, who proposed the coverage requirement as an amendment to sweeping health legislation, said it could save millions of lives.
“My amendment would eliminate one of the major barriers to care by getting rid of high co-payments and deductibles,” Ms. Mikulski said. “It does not tell women, ‘You will have a mammogram at 40.’ It says, ‘You will have access to that mammogram if you and your doctor decide it’s medically necessary or medically appropriate.’”
Senator John McCain of Arizona led efforts by Republicans to defend Medicare against what they described as a stealthy raid by Democrats.
By a vote of 58 to 42, the Senate rejected Mr. McCain’s motion to send the bill to the Finance Committee with instructions to strip out the Medicare savings, which total nearly a half-trillion dollars over 10 years. The savings, along with new taxes and fees, help offset the cost of providing coverage to millions of people who are now uninsured.
In response to Mr. McCain’s proposal, Democrats scrambled to show their commitment to Medicare. The Senate voted 100 to 0 to adopt an amendment, proposed by Senator Michael Bennet, Democrat of Colorado, making it clear that guaranteed Medicare benefits would not be taken away as a result of the legislation.
With the votes Thursday, the Senate broke a logjam over how to handle the first amendments to the health care bill. But Senate Democrats still face a long road ahead. They are struggling to reach agreement among themselves on a few major issues, including creation of a government insurance plan to compete with private insurers.
The Senate debate could last for weeks. The latest votes highlighted the challenge Democrats face in trying to hold on to one of their centrists, Senator Ben Nelson of Nebraska. He sided with Republicans in three of the four roll-call votes Thursday.
Under Ms. Mikulski’s proposal, a federal agency, the Health Resources and Services Administration, would develop “comprehensive guidelines” recommending preventive care and screenings for women, and insurers would have to cover the services without any cost-sharing.
Ms. Mikulski said the services would include screenings for breast, cervical, ovarian and lung cancer, heart disease and diabetes, as well as postpartum depression and domestic violence.
The underlying bill would eliminate deductibles and co-payments for many preventive services for men, women and children. But Ms. Mikulski said it did not adequately address the “unique needs” of women.
Senator Olympia J. Snowe, Republican of Maine, was a co-sponsor of the Mikulski amendment. She and two other Republican senators, Susan Collins of Maine and David Vitter of Louisiana, joined 56 Democrats and 2 independents in voting for the amendment. Two Democrats, Russ Feingold of Wisconsin and Mr. Nelson, voted no.
Mr. Feingold said the Senate should have found a way to pay the cost of Ms. Mikulski’s proposal, estimated at $940 million over 10 years. Mr. Nelson said the amendment should have explicitly excluded abortion from the definition of preventive care.
The Senate action was prompted, in part, by the recent furor over breast cancer screening recommendations from a federal advisory panel. In a report last month, the panel, the United States Preventive Services Task Force, said it “recommends against routine screening mammography in women age 40 to 49.”
Democrats accepted Mr. Vitter’s proposal to guarantee free breast cancer screenings for women 40 to 49, as well as those 50 and over.
The advocacy arm of the American Cancer Society supported the Mikulski amendment and opposed a Republican alternative, which would have required insurers to inform patients of screening guidelines issued by professional groups like the American Society of Clinical Oncology and the American College of Obstetricians and Gynecologists.
Ms. Mikulski said the Republican alternative was “too tepid and too limited.”
Daniel E. Smith, president of the American Cancer Society Cancer Action Network, said the Republican proposal would not eliminate co-payments and deductibles that could deter some women from seeking preventive care.
If the Senate had accepted Mr. McCain’s proposal on Medicare, it would have killed the bill in its current form.
The bill would slow the growth of Medicare spending by trimming payments to hospitals, nursing homes, private Medicare Advantage plans and many other health care providers.
President Obama and Senate Democrats said the savings could be squeezed out of Medicare without hurting beneficiaries. Indeed, they said, the bill would improve benefits and add five years to the life of the Medicare trust fund, so the fund would be exhausted in 2022, rather than 2017.
But Republicans said they were doing more to protect older Americans.
“Let’s save seniors who have paid into the Medicare program their whole lives from these damaging cuts,” Mr. McCain said. “Let’s not use Medicare savings to fund a whole new $2.5 trillion entitlement program.”