President Bush on Sunday enthusiastically endorsed a tentative agreement on a new prescription drug benefit for older Americans. "I will be actively pushing the bill" in the coming days to ensure passage, he said.

Republican congressional leaders on Saturday announced the breakthrough on the legislation, which would mark the largest ever expansion of Medicare (search).

Their agreement on principles still requires approval of House and Senate negotiators and then votes in the full House and Senate. The deal includes a proposal to have traditional Medicare compete directly with new private insurance plans and a plan to encourage employers to maintain drug coverage for retirees, officials said.

"I urge the members of the House and Senate to take a look it, vote it, get it to my desk as soon as possible," Bush said upon his arrival at the White House after a weekend at Camp David (search).

"I know I will be actively pushing the bill because it conforms to the principles I laid out, of prescription drugs for our seniors, choice for seniors, accountability for the Medicare plan," the president said.

But a leading Democrat, Sen. Edward M. Kennedy of Massachusetts, told CBS' "Face the Nation" that he did not think the bill would pass the Senate.

Bush said interest groups "are getting mobilized" on the legislation and will help him and others overcome what appears to be some Democratic opposition to give "a good chance" of passage.

"It's a good piece of legislation," he said. "It is a complex piece of legislation — after all, we're changing a Medicare system that has been stuck in the past for a long period of time."

But one of the Democrats who joined the negotiations, Sen. Max Baucus (search) of Montana stressed that the deal would remain tentative until lawmakers had a chance to see the details.

Senate Democratic leader Tom Daschle said the plan appears to lack a way to control drug prices. "I think we need to do more to leverage lower prices, and I think government can play a role," he told "Fox News Sunday."

He expressed concern about the 2 million to 3 million people he said could list their retirement drug benefits under the plan. Also, the South Dakota Democrat 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."

The agreement ends months of negotiations over the drug benefit and a broad reworking of the Medicare program to give private insurers a new large role in health care for 40 million older and disabled Americans.

Many of the House and Senate negotiators have been meeting for months in search of a compromise.