The White House has shared so little information about its proposal to redesign Medicare that none of the lawmakers most crucial to its prospects in Congress has decided whether to endorse the plan, which President Bush will sketch out next week.

Senate Majority Leader Bill Frist (R-Tenn.), a close ally of Bush and longtime advocate of restructuring Medicare, is withholding judgment until he learns more details from the White House, according to congressional sources. The House's prime Medicare expert, Ways and Means Committee Chairman Bill Thomas (R-Calif.), received an administration briefing this week but has not indicated whether he has been able to make up his mind.

And Sen. John Breaux (La.), one of the few Democrats in Congress who shares Bush's basic faith in using private health plans to treat the elderly, has not been consulted lately at all. "We don't know what it is exactly," an aide to Breaux said yesterday of the plan.

Sources inside and outside the administration said this week that the White House is developing a proposal that would fundamentally redesign the 1960s federal program that provides health insurance to 37 million older Americans. Under the plan, the government would begin to help people on Medicare pay for prescription drugs. But the drug coverage would be available mainly -- and perhaps only -- to people willing to join a new version of Medicare that would rely on health maintenance organizations, physicians' networks and other managed care.

Medicare is to be a significant theme of the annual State of the Union address that Bush delivers Tuesday night, and he will travel to Michigan the next day to promote the issue, which the White House regards as a central element of its domestic agenda this year. His public remarks are likely to be somewhat general, because aides are still finishing the plan.

While the plan's potential supporters remained silent, skeptics in both political parties yesterday made clear that the road toward Medicare changes will be rocky.

Democrats and their ideological allies immediately criticized the White House's intentions. They accused Bush of coercing the elderly into managed care, even though the administration envisions a subtler strategy: trying to entice people on Medicare to join private health plans by offering better coverage, including more preventive care and limits on high expenses as well as the drug coverage.

"The president should go back to the drawing board, scrap his HMO drug plan, and write a Medicare drug plan," said Senate Minority Leader Thomas A. Daschle (D-S.D.). The ranking member of the Senate's health committee, Sen. Edward M. Kennedy (D-Mass.), called the pending proposal "an outrageous attempt to force senior citizens to give up their family doctor and join HMOs."

If Democrats were caustic, Republicans, particularly in the Senate, made clear in subtler ways that the GOP caucus would not uniformly support Bush's ideas.

Moderate Republicans, including several members of the Senate Finance Committee, which handles Medicare, have historically been reluctant to make broad changes that would place the program mainly in the hands of private health plans, which competed for Medicare patients. Others in the GOP are reticent for a separate reason: They favor tilting the program toward the private market, but they believe the government cannot afford to make drug coverage available to everyone on Medicare, preferring to target such benefits to elderly people who are poor.

A spokesman for Sen. Olympia J. Snowe (R-Maine), one of the Finance Committee moderates, said she "clearly would have concerns about an approach that required entry into managed care in order to receive prescription drugs."

Finance Committee Chairman Charles E. Grassley (R-Iowa) issued an equivocal statement that said he welcomed Bush's ideas and would consider "all options" but stopped short of praising the new version of Medicare the White House envisions. Instead, Grassley said Medicare needed to be modernized to include drug coverage, preventive care and other new benefits.

Even as public debate over the Medicare changes began, White House officials yesterday continued their refusal to discuss their proposal, in an effort not to upstage the timing and settings they have planned for Bush to announce it himself. Asked about the plan, White House press secretary Ari Fleischer repeated broad statements Bush has made before about his goals for Medicare. "The president will . . . make it a top priority to create a modernized Medicare that includes prescription drug coverage for seniors," Fleischer said. "And that will have a variety of options, a variety of choices."