The Senate cast its first votes on remaking the nation's health care system Thursday, approving an amendment to safeguard coverage of mammograms and preventive screening tests for women under a revamped system.

The 61-39 vote on a provision by Democrat Barbara Mikulski of Maryland and Republican Olympia Snowe of Maine was the first substantive ballot in an acrimonious debate that promises to go on for weeks, the legislative equivalent of trench warfare.

The vote came after three days of angry debate in which Democrats accused Republicans of stalling to try to kill the bill, and Republicans protested that they were only exercising their right to give the complex legislation full scrutiny.

The first vote was held under a special agreement requiring 60 votes to prevail. The outcome underscored the fragility of the coalition Democrats are counting on to move President Barack Obama's signature issue.

Votes on Medicare were scheduled for Thursday afternoon.

After that will come an amendment to restrict abortion funding, except in cases of rape, incest or to save the life of the mother. Drafted by an abortion opponent — Democratic Sen. Ben Nelson of Nebraska — it looms as perhaps the biggest challenge for the Democrats.

Though Democrats have 60 votes in the Senate, two Democratic senators voted against the Mikulski amendment — Russ Feingold of Wisconsin and Nelson. The measure was saved by three Republicans voting in favor — Snowe, David Vitter of Louisiana and Susan Collins of Maine.

Thursday's vote followed the heated controversy over a government advisory panel's recent recommendation that routine mammograms aren't needed for women in their 40s. Although the advisers' recommendation was nonbinding, it prompted fears that the health care legislation would usher in an era of rationing.

The Mikulski amendment gives the health and human services secretary authority to require health plans to cover additional preventive services for women. The Congressional Budget Office said the amendment would cost $940 million over a decade

Mikulski said her amendment would guarantee that decisions are left to women and their doctors, not placed in the hands of government bureaucrats or medical statisticians. She accepted a modification to her amendment by Vitter that would specifically prevent the controversial recommendations on mammograms from restricting coverage of the test.

However, Republicans said that Mikulski's amendment still left too much discretion to the HHS secretary. A competing amendment by Sen. Lisa Murkowski, R-Alaska, would prevent the government from using the recommendations of outside advisers to deny coverage of preventive services, including mammograms and Pap tests. It was defeated by on a vote of 41-59.

Votes scheduled later Thursday on Medicare go to the heart of seniors' concerns that cuts from the program used to finance coverage for the uninsured will undermine the quality of care for the elderly.

Sen. John McCain, R-Ariz., is proposing to strip more than $400 billion in Medicare cuts from the nearly $1 trillion bill, and send the whole measure back to committee so a different financing scheme can be found. A competing amendment by Sen. Michael Bennet, D-Colo., underscores that no benefits in traditional Medicare will be cut by the legislation. It also clarifies that the Medicare cuts — amounting to a 2 percent slowdown in spending — will extend the life of the program and lower premiums for beneficiaries.

Underscoring the political stakes, McCain, recorded "robocalls" in states that are home to key moderate Democrats asking voters to support McCain's amendment stripping the bill's Medicare cuts. The calls, paid for by the National Republican Senatorial Committee, targeted Nelson, Bennet and Blanche Lincoln of Arkansas.

"On Monday, I introduced the first Republican amendment to the massive health care bill, which would send the bill back to the Senate Finance Committee and stop the Democrats from cutting vital Medicare coverage for our seniors. I need Sen. Blanche Lincoln to join me in this effort," McCain says in the call heard by Arkansas residents.

He asks them to go to an NRSC Web site and sign a petition to Lincoln "urging her to join my effort to fight a Washington, D.C., government takeover of your health care."

The scripts in the other states were identical. Another call by a live operator was heard by voters in North Dakota, which prohibits robocalls, and it delivered the same message, targeted at Sen. Byron Dorgan.

If Democrats prevail in Thursday's votes, they face a make-or-break test on abortion funding.

While most Senate Democrats support abortion rights, they need the votes of a handful of abortion opponents in their party to pass the health care bill. Catholic bishops have been adamant in saying they will not accept any compromise that in their view would expand federal funding for abortion. Liberals in the House gave in under pressure, but they're determined to make a stand in the Senate.