President Bush on Sunday enthusiastically endorsed a tentative agreement on legislation to give older Americans a new prescription drug benefit (search), part of sweeping changes to the government-run Medicare program.

"I will be actively pushing the bill" in the coming days to ensure passage, Bush said, leading the Republican effort to overcome anticipated resistance from Democrats and objections from some Republicans.

Republican congressional leaders on Saturday announced the breakthrough on the legislation, which would mark the largest ever expansion of Medicare (search). Republicans hope to pass it by week's end.

Many details remained unknown Sunday as congressional staff worked on the cost of the bill -- which cannot exceed $400 billion over 10 years -- and its final composition.

But at its core, it would give millions of seniors prescription drug coverage for the first time for a premium of roughly $35 a month. At the same time, the bill would create a large new role for private insurers in health care for 40 million older and disabled Americans.

Senate Majority Leader Bill Frist (search), R-Tenn., said he thinks most lawmakers will be persuaded to support the bill because of the drug benefit and the ability of seniors to choose their health plans, including remaining in traditional Medicare.

"If you're satisfied with what you have today, you don't have to change," Frist said Sunday at a Capitol news conference to give an outline of the legislation.

Frist said AARP, the 35-million member seniors' group, specifically endorsed compromises on final sticking points in the bill, including efforts to encourage employers to maintain drug coverage for retirees, direct competition between traditional Medicare and private plans, and providing more money to low-income seniors.

AARP chief executive William Novelli said the group would make no decision on an endorsement until it reviews final details of the legislation.

Other interest groups, including the American Medical Association and the trade organization for the managed care industry, announced their support the legislation.

Sen. Max Baucus, D-Mont., one of two Democrats who took part in months of negotiations, said the bill "is not perfect from anyone's perspective, but it's good legislation."

He said the bill would attract significant Democratic support in the Senate, even if some leading Senate Democrats, including Edward M. Kennedy of Massachusetts, oppose it.

"Many will reach the decision that this is better to pass than not to pass," Baucus said.

Kennedy and Senate Democratic leader Tom Daschle of South Dakota raised warning flags Sunday that the bill would do more harm than good.

Kennedy said on CBS' "Face the Nation" that he did not think the bill would pass the Senate. Daschle, on "Fox News Sunday," said, "We may actually be coercing up to 10 million people into an HMO system that could mean dramatically higher costs ... in premiums for Medicare."

But in overcoming the final obstacles to the bill, negotiators said they reached a compromise on proposal to require traditional Medicare to compete with private insurance plans and found a way to reduce by half the number of retirees who would lose employer-based coverage once the new drug benefit kicked in.

Competition would begin in 2010 in six metropolitan areas and last no more than six years. Extending it beyond that would require congressional approval.

Sen. John Breaux, D-La., the other Democratic negotiator, said the bill would cap annual premium increases for seniors who remain in traditional Medicare at 5 percent a year, responding to criticisms that premiums would soar for those who did not move to private plans.

And Baucus said he disagreed with those who say the temporary program would harm traditional Medicare.

In the House, a conservative Republican said he is disappointed with the agreement. Competition "has been watered down to a demonstration project that does not even begin until 2010," Rep. Jeb Hensarling, R-Texas, said. "I frankly do not know how we can stave off bankrupting the Medicare system without this essential reform."

Hensarling was one of several conservatives who voted reluctantly for the Medicare bill passed by the House in June by a one-vote margin.

He said he and other conservatives would reserve judgment until they see the final bill.

The legislation includes several provisions designed to meet conservatives' demands. These include a new health-related tax break for individuals with a high-deductible health insurance policy; a requirement for higher wage-earners to pay more for the out-of-hospital coverage they receive under Medicare Part B; and a requirement for Congress to review Medicare's costs if they rose to unexpected levels.

The agreement also contains at least $25 billion for rural doctors and hospitals.

It would essentially leave unchanged the ban on importing prescription drugs from Canada, although it would authorize a study to determine what it would take to guarantee the safety of imported drugs.