A day after his State of the Union address, President Bush took his agenda for the year on the road, starting with ambitious and expensive changes to Medicare. But the White House remained conspicuously silent about exactly how it wants to redesign the insurance program for the elderly even as the president traveled to the Midwest to begin selling the idea.

Administration officials who accompanied Bush here today repeatedly skirted a central question that is drawing fire from Democratic critics: Does the White House intend to make popular new drug benefits available to older Americans who remain in the original form of Medicare -- or only to those willing to sign up for a new version of the program that relies on managed care?

Of the many intricacies involved in redesigning a 1960s health program that covers nearly 40 million elderly people, the question of which people would qualify for drug benefits has singular political and budgetary significance. Patients and congressional Democrats have made plain they will not accept any plan they believe would corral patients into managed care. On the other hand, health policy analysts predict that the government may be unable to afford a plan that would give substantial subsidies for medicine even to people who stay in the traditional fee-for-service version of the program -- the version to which nearly nine in 10 people in Medicare now belong.

Last week, sources said that Bush will propose a fundamentally new form of Medicare that would try to entice people into private health plans by offering more benefits, including drug coverage. But they said the president had not decided whether patients could keep the original version of the program and get help with drug expenses. Yesterday, White House aides were far less clear, emphasizing mainly that the administration was eager to promote new alternatives, including health plans and doctor networks.

With such crucial details unresolved -- or at least, unspoken -- the White House is in an awkward spot, trying to tamp down the outpouring of opposition without spelling out its plan.

Here in western Michigan, a state that he narrowly lost in the 2000 election and hopes to win next year, Bush stood on an auditorium stage in front of a large backdrop that said, "Strengthening Medicare." He devoted three minutes of a 42-minute speech to the topic.

Saying that Medicare "has been used as a political football," Bush reiterated his appeal from Tuesday night for Congress to "put aside politics and to make sure the Medicare system fulfills its promise to our seniors," including coverage of prescription drugs. Bush said he would spend $400 billion to improve the program and restated his longtime pledge that people in the Medicare system will not be forced to change.

The remainder of Bush's remarks spanned the themes from the night before. They included further tax cuts, public education, limits on medical malpractice lawsuits, a voucher proposal that would provide subsidies for drug treatment by religious groups, and the increasingly biting language Bush is using to denounce Iraqi President Saddam Hussein.

After the speech, a senior administration official briefed reporters on a half-dozen new initiatives that Bush had proposed in the State of the Union address -- excluding Medicare. Pressed about the health insurance program, the official, speaking on condition of anonymity, said Bush probably would send Congress a detailed plan specifying the Medicare changes he seeks. But neither that official nor White House press secretary Ari Fleischer would predict when that would occur.

Asked repeatedly whether patients who wanted to stay in the traditional Medicare program could get drug coverage, Fleischer did not address the question directly. But when asked whether patients would have to join some kind of managed-care plan, he replied, "No, no, no, no."

House Majority Leader Tom DeLay (R-Tex.) reinforced that message on Capitol Hill. Asked whether a Medicare patient would need to join a health maintenance organization to qualify for help in paying for drugs, Delay said: "Whoa, whoa, whoa, whoa. That's absolutely false."

Those answers, however, would not preclude one avenue that sources have said the administration is contemplating: establishing drug subsidies and other expanded benefits for patients who are willing to join a "preferred provider organization." That type of managed care, which has become more widespread in recent years, steers patients toward physicians in a limited network. Bush and his aides have said they want Medicare to resemble the range of insurance alternatives available to federal employees; the handbook on insurance plans for such employees lists such networks among its fee-for-service options.