The Bush administration’s point person for the giant Medicare and Medicaid health programs, Dr. Mark McClellan, is stepping down, the White House said on Tuesday.

McClellan said in a letter to staffers that he would leave the Center for Medicare and Medicaid Services by early October, after a transition period. No immediate announcement was made about his likely replacement.

“This was a hard decision, because this is the most exciting and rewarding place that anyone could ever work,” McClellan, 43, wrote in an e-mail sent to staff.

“But I’ve been in government service for much longer than my family and I had ever expected or prepared for, and after almost six years in this administration plus service in the previous administration as well, I’m looking forward to more dinners at home,” he wrote.

McClellan oversaw the rollout earlier this year of the new Medicare prescription drug program for older Americans. CMS, part of the Health and Human Services Department, accounts for about a fifth of the U.S. budget, with more than $500 billion worth of spending in 2006.

The prescription drug program, created by legislation that President George W. Bush fought to push through the U.S. Congress, got off to a difficult start since many older Americans were bewildered by the wide array of choices.

Democrats have criticized the glitches but Republicans hope the creation of the new benefit will help them in their push to retain control of Congress in the November midterm elections.

“We’ve worked with partners to get 90 percent of people with Medicare into drug coverage, and we’ve implemented the biggest transition in drug coverage ever in this country,” McClellan wrote.

“We’ve found and fixed start-up problems, we are delivering coverage at a cost at least 25 percent less than had been expected, and we are seeing beneficiary satisfaction rates of over 80 percent. For 2007, the benefit costs are going down further, and many drug plans will have lower premiums and options for enhanced coverage as well.”

A medical doctor with a PhD in economics, McClellan has headed the Center for Medicare and Medicaid Services since March 2004 and has served in the Bush administration for five years. He has been commissioner of the Food and Drug Administration and a member of Bush’s Council of Economic Advisers.

McClellan is the brother of Bush’s former press secretary, Scott McClellan, who left the administration earlier this year. Their mother is Texas State Comptroller Carole Keeton Strayhorn, who is running as an independent for governor of Texas.

McClellan said he hoped a planned transformation of Medicare would continue.

“We are implementing the most important reforms in more than two decades in hospital payments, and important reforms in ambulatory surgery and many other types of care,” he wrote in his e-mail.

“We’ve also worked to lay the foundation for a fundamentally better, more sustainable payment system for physicians. We are collaborating to move to paying for what we really want, and what providers want to deliver: better care at a lower overall cost, not just more services.”

The Center for Medicare and Medicaid Services administers Medicare, the joint state-federal health insurance plan for the elderly and disabled, Medicaid, which provides benefits mainly to the poor, and the State Children’s Health Insurance Program.

The children’s program covers about 4 million people. Medicare has 41 million beneficiaries and Medicaid has 55 million clients.

The Dallas Morning News reported that McClellan would take a position at the American Enterprise Institute, a conservative think tank.