Taking advantage of favorable cost estimates, supporters of Medicare prescription drug legislation (search) are working to close gaps in coverage under the bill before a showdown later this week in the Senate Finance Committee.

"We're going to have a $400 billion drug plan when it's all said and done," Sen. Charles Grassley, R-Iowa, chairman of the Senate Finance Committee said Monday. An early draft was estimated to cost $359 billion over 10 years, and $8 billion less than that after taking unidentified offsetting provisions into account - meaning the bill's architects had tens of billions in additional funds to devote to the measure.

Grassley declined to say precisely how the bill would change, but several officials said they expected the available funds would be used in part to reduce the burden on seniors with high drug costs, and possibly to increase help for lower-income beneficiaries as well.

In addition to providing a prescription drug benefit, the legislation would establish a new managed care option for Medicare. Seniors would be offered the chance to enroll in Preferred Provider Organizations (search) that the administration says would help modernize the program while putting its finances on a stronger foundation.

Grassley and Sen. Max Baucus, D-Mont., the senior Democrat on the committee, arranged to release their legislation publicly Tuesday.

While Baucus and a few Democrats support the measure, Senate Democratic leader Tom Daschle of South Dakota has been sharply critical of it. At the same time, Sen. Edward Kennedy, D-Mass., a liberal and strong voice in his party on health care issues, has praised the effort as a good beginning but called for changes.

Daschle has said repeatedly he doesn't expect to filibuster the proposal, a decision that strongly improves the chances for passage of Medicare prescription drug legislation after years of partisan gridlock.

Health and Human Services Secretary Tommy Thompson (search) traveled to the Capitol on Monday, and leading Republicans said afterward they had heard words of encouragement.

"The president is very high on the process we've got going here, without agreeing on every detail of what we're doing. I find this very helpful," Grassley told reporters afterward.

Asked whether Bush would sign the measure if it cleared Congress as drafted, he replied, "I believe he would."

Senate Majority Leader Bill Frist, R-Tenn., told reporters, "They recognize that the bill as being presented is different than the president's bill, but underscored that it meets all of the president's principles that have been laid out to date."

One area of difference revolves around Bush's proposal to give a better drug benefit to seniors willing to leave traditional Medicare and enroll in PPOs.

The Senate bill has equal benefits for both groups, and Thompson said during the day he was "not really at this point in time" seeking a change in that provision in the legislation.

Asked whether the administration was still pressing for the president's proposal, Grassley said, "No, it doesn't look like it."

Pending overnight changes, aides have said that beneficiaries enrolled in traditional Medicare could purchase stand-alone drug coverage for a premium of roughly $35 a month. The plan would carry a deductible of $275, after which the individual would be required to pay 50 percent of the bill until total costs reached $3,450. From that point until costs reached $5,300, the individual would pay 100 percent of the bill. Beyond that level, insurance would pay 90 percent, and the individual 10 percent.

The government would subsidize expenses for low-income beneficiaries.