Washington ? Unflinching on a critical first test, Senate Democrats closed ranks Thursday behind $460 billion in politically risky Medicare cuts at the heart of health care legislation, thwarting a Republican attempt to doom President Barack Obama’s sweeping overhaul.

The bid by the bill’s critics to reverse cuts to the popular Medicare program failed on a vote of 58-42, drawing the support of two Democratic defectors. Approval would have stripped out money needed to pay for expanding coverage to tens of millions of uninsured Americans.

The broader legislation aims to extend health coverage to 31 million who now lack it, while barring insurance industry practices such as denying coverage on the basis of pre-existing medical conditions. Though the overhaul is estimated to cost about $1 trillion over a decade, the Congressional Budget Office has said it would cut federal deficits by $130 billion over that period, and probably reduce them further in the 10 years beyond that.

“Our bill does nothing to reduce guaranteed Medicare benefits,” said Sen. Max Baucus, D-Mont., as several fellow Democrats accused Republican critics of making false claims of potential harm during three days of debate.

The AARP supported the 10-year package of cuts in projected spending, giving Democrats political cover for their decision to pare back subsidies to private Medicare plans as well as payments to hospitals, hospices, home health agencies and other providers.

Republicans disagreed vigorously. “Medicare is already in trouble. The program needs to be fixed, not raided to create another new government program,” said the party’s leader, Sen. Mitch McConnell of Kentucky.

The Medicare vote came not long after the Senate backed a guarantee for all insured women age 40 and older to receive mammograms with no out-of-pocket costs. The breast cancer screening test would be included in an array of preventive measures that insurance plans would be required to cover. The proposal cleared on a near party-line vote of 61-39, one more than the 60 needed for passage. It essentially wiped out a federal advisory committee recommendation to defer routine mammograms until women reach the age of 50.

The day’s votes were the first since the Senate’s health care debate began on Monday, and demonstrated the ability of Democrats to move ahead in the face of implacable Republican opposition.

At the same time, Democrats worked in private meetings to settle controversies within the party that are still standing in the way of the bill’s passage. The most contentious of these involves proposals for the government to sell insurance in competition with private companies, an approach supported by liberals but opposed by most Democratic moderates and conservatives.

“Our caucus is now in the process of negotiating with ourselves because we need all 60 of us to get this done,” said Sen. Mary Landrieu, D-La., after one closed-door meeting. Senate procedures require 60 votes to overcome Republican delaying tactics designed to kill the bill.

Majority Leader Harry Reid, D-Nev., said he expected all such issues to be worked out soon, but he did not specify precisely when.