Washington ? The House of Representatives is set to vote today on a final version of Medicare prescription drug coverage. House and Senate negotiators slightly sweetened the terms of the drug subsidy for consumers Thursday before voting to send the measure to both chambers for final votes.

The House vote appears likely to be close and to split largely along partisan lines, but Republican leaders probably would not have scheduled the vote for today if they were not confident of victory.

Senate passage appears certain early next week as Democratic senators began lining up behind the largely Republican-written bill. The legislation would mandate the largest increase in Medicare coverage and spending in nearly four decades. For the first time, it would help seniors pay for prescription drugs and it would dramatically expand the role of private insurers in Medicare.

Under the drug benefit — the bill’s centerpiece — seniors would pay a $250 deductible and $35 a month premium for 75 percent coverage of their prescription medicines up to total drug cost of $2,250. The federal government would pick up 95 percent of prescriptions after total drug costs reached $5,044.

Low-income seniors would get a better package. Seniors with incomes under 135 percent of the poverty line would pay no deductible, no premiums and a tiny co-payment for each prescription.

The Congressional Budget Office placed the bill’s final cost at $394 billion over 10 years.

Democratic critics complain that the bill’s terms creating an expanded role for private insurers will undermine traditional Medicare. Conservative critics say the bill does not do enough to contain Medicare costs and encourage greater competition with private plans.

But many lawmakers are swayed by provisions that would expand spending on hospitals, doctors and other health care providers, particularly in rural areas.

Also influencing the outcome was the bill’s endorsement by the AARP, the seniors’ lobby that counts 35 million members.