President Obama's top Medicare official has resigned in the face of Republican pledges to block his confirmation in the Senate. Center for Medicare and Medicaid Services Administrator Donald M. Berwick notified colleagues Wednesday that he will step down Dec. 2, nearly a month before the expiration of his recess appointment. The White House will nominate Marilyn Tavenner, Medicare's deputy administrator, as his replacement. "Don Berwick did outstanding work at CMS," White House deputy press secretary Jamie Smith said Wednesday. "It's unfortunate that a small group of senators obstructed his nomination, putting political interests above the best interests of the American people." Obama nominated Berwick to run Medicare in April 2010. In July 2010, with no confirmation hearing scheduled, the president appointed him to the job while Congress was in recess. As a recess appointment, Berwick's term was to expire Dec. 31. Earlier this year, 42 Republican senators signed a letter pledging to block his confirmation, effectively ending any chance of him serving beyond 2011.