HARRISBURG — A top aide to Gov. Tom Corbett said expanding Medicaid eligibility in Pennsylvania under a sweeping federal health care law probably would not take effect before January 2015, even if the governor embraces the idea that would provide taxpayer-paid health care insurance to hundreds of thousands of residents.

Beverly Mackereth, Corbett’s Department of Public Welfare chief, told a Pittsburgh newspaper the administration would need until 2015 to negotiate with the federal government and create the program.

Still, Mackereth told the newspaper the timeline remains ambiguous and is predicated on negotiations, agreements and the creation of the program going well. “And those are huge ‘ifs,”’ she said.

The federal government would foot nearly all of the cost of providing health care to hundreds of thousands of additional Pennsylvanians by widening Medicaid’s income eligibility guidelines to 133 percent of the federal poverty level, or about $31,300 for a family of four. The federal law initially promises 100 percent funding to expand Medicaid eligibility beginning Jan. 1, 2014, primarily to extend insurance to low-income working adults.

After three years, the federal government begins to drop its portion of the cost to 90 percent.

Democratic lawmakers say waiting until 2015 would mean the loss of $4 billion in 2014 for Pennsylvania and vowed to keep pushing Corbett to expand Medicaid next year.

“I think Pennsylvania citizens would have a big problem — a big problem — if $4 billion targeted toward Pennsylvania found their way somewhere else across the country, especially in a tight budget situation,” Sen. Vincent Hughes, D-Philadelphia, said Monday.