WASHINGTON - Republican congressional leaders are hearing it from both sides: Their emerging Medicare drug legislation threatens to undermine Medicare. Or it doesn't go far enough in changing the government-run health care program for the elderly and disabled.

They're betting it's just the right blend to attract a majority in the House and Senate.

But even as two major hospital groups endorsed the plan and President Bush called for the legislation soon, a growing chorus of Senate Democrats criticized the bill Thursday.

"This program is untested. It's untried. ... It's playing roulette with the lives of senior citizens," said Sen. Edward M. Kennedy, D-Mass., taking aim at a feature that would put traditional Medicare in direct competition with new private insurance plans beginning in 2010.

Seven Republican senators, led by Olympia Snowe of Maine, joined 30 Democrats and an independent in signing a letter that urged the removal of the competition provision.

More exasperating, from the GOP leaders' perspective, was the complaint by Rep. Bill Thomas, R-Calif., the lead House negotiator, that the plan did not go far enough. "It's guaranteed not to work," he said of the plan, while one of its authors, Senate Majority Leader Bill Frist, R-Tenn., stood by his side.

Thomas' objections frustrated the leaders' desire to win the informal approval of the negotiators. But GOP leadership aides said the legislation remained on track to be brought to a vote next week, and Democrats stopped short of threatening a filibuster that could delay or derail passage in the Senate.

At its core, the emerging bill would provide the elderly a prescription drug benefit beginning in 2006, at a cost of $35 a month. Low-income patients would receive subsidies to defray the cost. At the same time, the plan would encourage private insurance companies to offer coverage under preferred provider organizations for seniors willing to opt out of traditional Medicare.

While lawmakers and interest groups argued in Washington, the president was in Florida, warning against "more deadlock" and urging Congress to act "for today's seniors but also for tomorrow's retirees."

In his remarks, Bush made no specific reference to the proposal brokered by Frist and House Speaker Dennis Hastert and embraced thus far by only two Senate Democrats.

Sources familiar with negotiations said Republicans had agreed during the day to concessions to delay the beginning of their plan for competition, and to provide further protection against the type of premium increases that Kennedy and other critics forecast would spell the doom of Medicare.

Rather than start in 2007, the program would begin in 2010, these sources said. And instead of limiting premium increases to 10 percent annually for seniors remaining in traditional Medicare, the cap would be 5 percent. Unresolved was the number of recipients to be covered.

Already, the special interests were beginning to line up.

The American Hospital Association and the Federation of American Hospitals announced their support of the bill, pleasing Republicans.

The Committee to Preserve Social Security and Medicare, Families USA and the Alliance for Retired Americans joined Kennedy and other Democrats in opposition at a Capitol news conference.

GOP officials said they expected to gain the approval of the politically influential AARP, although the group's chief executive suggested the plan for direct competition in four metropolitan areas and one region be scaled down. "That sounds big," William D. Novelli said in an interview, adding the issue was "an example of what others should negotiate."

The plan blessed by Hastert and Frist calls for a three-year program. The Department of Health and Human Services could approve a three-year extension of the program, but after that, it would expire without action by Congress.