WASHINGTON — A pillar of the Democrats' program tumbled Wednesday when the Senate blocked a proposal to let Medicare negotiate lower drug prices for millions of Americans, a practice forbidden by law.

Democrats could not muster the 60 votes needed to take up the measure in the face of staunch opposition from Republicans. The opponents said private insurers and their agents, known as pharmacy benefit managers, were already negotiating large discounts for Medicare beneficiaries.

Fifty-five senators, including six Republicans, supported a Democratic motion to limit debate and proceed to consideration of the bill; 42 senators voted against it. The Senate had a brief debate on the merits of the bill, which is a priority for Congress' new Democratic majority.

Republicans framed the issue as a choice between government-run health care and a benefit managed by the private sector. The benefit is delivered and administered by private insurers under Medicare contracts.

Sen. John Cornyn, R-Texas, denounced the bill as "a step down the road to a single-payer government-run health care system."

Democrats said they were merely trying to untie the hands of the secretary of health and human services so he could negotiate on behalf of 43 million Medicare beneficiaries.

"The Department of Veterans Affairs is able to negotiate for lower-priced drugs," said the Senate majority leader, Harry Reid, D-Nev. "HMOs can negotiate. Wal-Mart can negotiate. Why in the world shouldn't Medicare be able to do that?"

A 2003 law prohibits Medicare from negotiating or setting drug prices or establishing a uniform list of covered drugs, or formulary.

Reid said Democrats fell short because of "the power of the insurance industry and the pharmaceutical industry," which spent hundreds of thousands of dollars on lobbying and advertisements against the bill.

The vote also reflected ineffectual advocacy by Democrats, who were slow in responding to the vehement arguments of well-prepared Republican senators like Charles E. Grassley of Iowa.

"Private competition works," said Grassley, a principal author of the 2003 law. "The Department of Health and Human Services has had very little experience and a dismal track record" figuring out what to pay for drugs.

Big companies that offer the Medicare drug benefit, like Caremark and Medco Health Solutions, "have more market power than Medicare" because they negotiate for tens of millions of people in private plans, as well as for Medicare recipients, Grassley said.