HARRISBURG, Pa. — Gov. Tom Corbett is pressing the federal government for an exemption that he said will prevent about 70,000 Pennsylvania children in a state-subsidized health insurance program from having to switch to Medicaid, although a public interest law center challenged Corbett's claims and said the children will be better off under Medicaid.

Corbett wrote Thursday to U.S. Health and Human Services Secretary Kathleen Sebelius about his latest request, part of his effort to press her agency to make enough concessions to a federally funded expansion of Medicaid before he will change his mind and allow Pennsylvania to join it.

A Sebelius spokesman did not immediately comment on the letter. Community Legal Services of Philadelphia, a public interest law center that advocates for the poor, disputed Corbett's claims, including his core contention that some families will have to switch doctors and will have a smaller choice of doctors that they can visit.