BALTIMORE--CMS Administrator Mark McClellan, who has overseen Medicare's biggest expansion since the program's inception 40 years ago, announced Tuesday that he will resign his post early next month.

Taking the helm in 2004 after former Administrator Tom Scully left the position, the 43-year-old physician and economist has supervised the agency during rollout of Medicare's Part D prescription drug benefit--and as planning moves toward DME competitive bidding in 2007. He also has guided CMS during calls for Medicaid reform and, more recently, in implementing controversial provisions in the Deficit Reduction Act, including its 36-month cap on home oxygen rental.

McClellan told reporters his decision to resign from CMS was not related to a possible takeover by Democrats in the House or Senate after the November elections, which could result in oversight hearings on the new drug benefit. The drug program got off to a rough start, with beneficiaries complaining about problems obtaining their medications. However, McClellan said most beneficiaries are now satisfied with the program and that their costs are lower than originally projected.

In a statement, Senate Finance Committee Chairman Sen. Charles Grassley, R-Iowa, said he hoped McClellan's departure "doesn't cause delays in getting Part D snags fixed for beneficiaries and that he'll correct all known problems before he leaves." However, Grassley, whose committee oversees Medicare, added that McClellan "should get credit for a lot of behind-the-scenes work to make sure millions of Medicare beneficiaries get their prescriptions filled smoothly."

Other legislators were not so generous.

"I hope the next administrator places a renewed focus on oversight, shies away from partisan politics and works to protect and preserve Medicare for future generations," Rep. Pete Stark, D-Calif., said in a statement.

Before his appointment to CMS, McClellan served as commissioner of the Food and Drug Administration. He has also served as a senior health policy aide to President Bush and as a member of the White House Council of Economic Advisors. Previously, he worked at Stanford University as an associate professor and director of the Program on Health Outcomes Research, and was an attending physician for internal medicine at Stanford Health Services.

McClellan's successor has not yet been named, though insiders say there are several front-runners for the job, including Deputy CMS Administrator Leslie Norwalk; Herb Kuhn, director of the Center for Medicare Management at CMS; and Dennis Smith, director of the Center for Medicaid and State Operations at CMS. Because the current legislative session is winding down, an acting CMS director could be named to serve until the Senate holds a confirmation hearing on the new nominee.

The job includes administering a budget estimated at almost $600 billion a year on health care for approximately 90 million Americans through Medicare, Medicaid and the State Children's Health Insurance Program. McClellan's replacement will likely have to face questions about how to sustain Medicare as the nation's 78 million baby boomers move into eligibility and approach old age. There could also be continuing questions about the Part D drug benefit snafus, Medicaid reform plans and additional program cuts that many expect in the Bush administration's next fiscal year budget proposal.

McClellan, whose brother Scott McClellan is the former White House press secretary, said he is interested in taking a job with a think tank.