HARRISBURG — U.S. Sen. Bob Casey is asking Gov. Tom Corbett to embrace an expansion of Medicaid to ensure that federally funded health insurance is available to hundreds of thousands of Pennsylvania’s working poor when it becomes available next year.

In a Thursday letter, Casey warned Corbett that refusing the Medicaid expansion unnecessarily penalizes people who would otherwise have coverage beginning Jan. 1 if the federal government does not agree to changes in the program being sought by Corbett.

“Expanding Medicaid on Jan. 1, 2014, would allow you to pursue broader reforms without denying access to quality, affordable care to thousands of Pennsylvanians,” Casey wrote.

A Corbett spokesman said he was unaware of any plans to expand Medicaid in the state and instead encouraged Casey to help win federal approval of an alternative the governor’s office is proposing.

Corbett, a Republican, is a critic of Medicaid and as attorney general unsuccessfully sued to overturn President Barack Obama’s signature law that pledges federal funding to largely underwrite coverage for millions of Americans. Casey, a Democrat, supports the law.

In the meantime, Corbett is asking the Obama administration for permission to use federal Medicaid expansion dollars to pay the premiums for newly eligible adults to get private insurance in a new health care marketplace instead of being used to expand the traditional Medicaid coverage that typically pays lower reimbursements to doctors and hospitals.

The Corbett administration estimated that 520,000 more Pennsylvanians could get health insurance through the private market under broader Medicaid eligibility guidelines that include adults making up to 133 percent of the federal poverty level.