President Barack Obama announced Wednesday that he has nominated Marilyn Tavenner to serve as administrator of the Centers for Medicare and Medicaid Services (CMS), following reports that Donald Berwick would be stepping down in early December.

Tavenner, the former Virginia Secretary of Health and Human Resources under form Gov. Tim Kaine, had served as the principal deputy administrator CMS since February 2010 and as acting administrator from February 2010 to July 2010, according to the White House. Tavenner served as Berwick's principal deputy.

CMS, an arm of the Department of Health and Human Services, administers the Medicare and Medicaid programs which provide health care benefits to seniors, the disabled and low-income Americans. The entitlement programs have long been a target for overhaul by Republicans and others who say they are too costly and contribute to the rising federal deficit.

Berwick, a pediatrician and Harvard professor, was widely respected for his ideas on how to improve the health care system, became the most prominent casualty of the political wars over a health care overhaul law whose constitutionality will be now decided by the Supreme Court. A senior congressional official, speaking on the condition of anonymity, told the Associated Press that Berwick would tender his resignation on Dec. 2.

Berwick had been named to head CMS by Obama in July 2010  in a recess appointment, but 42 Republican senators announced their opposition earlier this year to his permanent appointment, enough to block it. His temporary appointment was to expire at the end of the year.

Berwick oversaw the drafting and rollout of major Medicare and Medicaid regulations that will begin to reshape the health care system, steering Medicare from paying for sheer volume of services and procedures and instead putting a premium on quality care that keeps patients healthier and avoids costly hospitalizations.

The Associated Press contributed to this report.