Dr. Donald Berwick, the man in charge of Medicare and at the heart of implementing President Obama’s health care law, faced a barrage of questions from House Republicans over concerns about the plan and on his own controversial past positions.

But Berwick mostly praised the embattled new law and sidestepped questions about his own opinions on government-run health care.

A main Republican concern about the law is that the $575 billion it requires in Medicare reductions will make it hard for seniors to find doctors and other providers...