McClellan said his agency is working with Social Security and a range of national and local groups to educate seniors in time for Part D's scheduled start.
"It takes a lot more than a few ads or speeches to help seniors make confident, informed decisions about their coverage," he said.
Still, the financial condition of Medicare and Medicaid remain a contentious issue. The Bush administration this week convened a commission charged with recommending ways to cut $10 billion in spending.
Consumers groups are deeply opposed to the cuts, saying they will damage the health care safety net relied upon by millions of low-income Americans.
"Tell your members of Congress: On Medicaid's birthday, don't cut it -- protect it," the consumer health group Families USA said Thursday in a statement.
Medicare's long-term finances also loom even as policy makers focus on reforming Social Security. AARP warned Thursday that beneficiaries' out-of-pocket premium costs are expected to keep rising as Medicare's costs go up in future years.
"As the nation looks for ways to keep Social Security solvent, it must protect its guaranteed benefit which will be vital not only to the economic security of older Americans, but to their health security as well," said John Rother, the group's director of policy.