President Barack Obama's new nominee for health secretary drew some early political fire from Republicans on Sunday in what could foreshadow a stormy election-year confirmation debate in the U.S. Senate over the future of the law known derisively as "Obamacare."

Two days after Obama nominated his budget director, Sylvia Mathews Burwell, to head the Department of Health and Human Services (HHS), Republican lawmakers alleged the new nominee could help the White House exert political control over health care enrollment numbers and other data indicating how well the reforms are working.

After a disastrous launch last year, the administration's enrollment drive has surpassed the most optimistic independent forecasts by signing up 7.5 million people in private health plans through new online insurance marketplaces.