HARRISBURG, Pa.—U.S. Sen. Bob Casey is asking Gov. Tom Corbett to embrace a Medicaid expansion to provide health insurance to hundreds of thousands of Pennsylvania's working poor.

In a Wednesday letter provided to The Associated Press, Casey warned Corbett that refusing a Medicaid expansion unnecessarily penalizes people who'd otherwise have coverage beginning Jan. 1.

Corbett, a Republican, is a critic of Medicaid and President Barack Obama's signature law that pledges federal funding to largely underwrite coverage for millions of Americans.

Casey, a Democrat, supports it.

Meanwhile, Corbett is asking the Obama administration for permission to use Medicaid expansion money to instead subsidize private health insurance for Pennsylvania's working poor. Casey says Corbett should expand Medicaid while he explores this avenue. A Corbett spokesman says he's not aware of any such plans.