Washington ? Just weeks before Congress adjourns for the year, the drive to add a prescription drug benefit to Medicare has bogged down in disagreement between Republicans and Democrats over efforts to privatize the program.

Other issues also divide the congressional negotiators trying to forge compromise legislation that would overhaul Medicare and cover some of the prescription drug costs of 40 million senior and disabled Americans.

But it is the stalemate over the administration-supported plan to have the invisible hand of the market — rather than the bureaucratic hand of government — determine prices and premiums that poses the greatest threat to the effort to expand Medicare benefits.

“That is virtually a showstopper,” Senate Minority Leader Tom Daschle, D-S.D., said Thursday of the privatization plan. Under that approach, health maintenance and preferred-provider organizations would compete directly for patients with traditional fee-for-service Medicare.

Almost three months after the House and Senate passed competing $400 billion Medicare bills, expectations for a successful compromise remain high. Indeed, the conference committee of congressional negotiators could produce a list of tentative agreements as soon as this afternoon.

But that won’t necessarily produce a final deal, some cautioned.

“Even what we think might be final may not be final,” said Sen. Charles E. Grassley, R-Iowa, vice chairman of the congressional conference committee.

Conference committee chairman Rep. Bill Thomas, R-Calif., meanwhile, has continued to push a framework that closely resembles the bill that passed the House by just one vote.

That action prompted 41 senators — enough to sustain a filibuster — to declare Thursday that such a bill would meet certain defeat.

In a letter to President Bush, the 39 Democrats, one Republican and one independent outlined their concerns and called on him “to provide the leadership necessary to achieve a bipartisan bill that can become law.”