Unable to repeal President Barack Obama's health care law, Republicans are trying to oust the official quarterbacking the overhaul of the nation's medical system.

In a letter released Thursday, 42 Republican senators asked the president to withdraw the nomination of Dr. Donald Berwick as Medicare administrator. That job carries major responsibilities under the health care law, such as setting up new insurance markets, expanding Medicaid to cover millions more low-income people, and revamping the way Medicare pays providers to reward quality instead of volume.

Republicans would need only 40 votes to block Berwick's confirmation by the full Senate, and the letter indicates that they have more than enough. The loss of Berwick, a well-known medical innovator and advocate for patients, would be a blow to the administration as it moves ahead with critical building blocks of Obama's health care remake.

The White House said Obama would not withdraw Berwick's nomination.

Sen. Orrin Hatch of Utah, ranking Republican on the committee that oversees Medicare, says Berwick lacks experience in insurance and that his past statements praising the British health care system mark him as a proponent of big government solutions to the nation's problems of soaring health care costs and dwindling coverage.

"Dr. Berwick's lack of experience in the areas of health plan operations and insurance regulation raise serious concerns about his qualifications for this position," said the senators' letter. "Withdrawing Dr. Berwick's nomination would be a positive first step in rebuilding the trust of the American people."

Berwick, a pediatrician by training and a Harvard professor, is widely respected in the medical community and among health care experts, including former Republican administrators of Medicare. But in the polarized politics of Washington, his past statements praising Britain's government-run national health care system became an instant source of controversy. Republicans accused him of advocating health care rationing, which Berwick denies.

Obama bypassed the Senate last year to install Berwick as a recess appointment. Under the rules, that means that unless Berwick can be formally confirmed, his appointment would run out at the end of this year.

The administration kept Berwick out of the public eye in the run-up to the congressional midterm elections. But this year, he has made the rounds of Capitol Hill, testifying before congressional committees. Rumpled and usually ready with a hands-on anecdote from his clinical experience, he stood up to hard questioning from Republicans.

Berwick is currently working on a major regulation that would allow hospitals and groups of doctors to band together to provide coordinated health care for Medicare patients. Incorporated in the health care law, so-called "accountable care organizations" would try to save taxpayers' money while improving patients' lives by trying to avoid unnecessary hospitalizations.