Thursday's announcement that Pennsylvania will expand its Medicaid program brings the country one state closer to the original expansion outlined under Obamacare. But because of the Supreme Court's 2012 decision making the expansion a voluntary program, there are still 23 states that haven't expanded public health insurance to all of their low-income residents.

The expansion in Pennsylvania will add about 500,000 low-income to adults to the Medicaid rolls. According to numbers from the Kaiser Family Foundation, about 281,000 of those people were falling into what's known as the "coverage gap"— people who don't qualify for Medicaid but also don't get subsidies for purchasing insurance on their own, either. About 4.5 million people across the country fall into this coverage gap, according to Kaiser.

State Medicaid programs generally cover some adult populations, like parents and disabled individuals, at varying income levels, but the Affordable Care Act's Medicaid expansion expands eligibility to all low-income adults earning up to 133 percent of the federal poverty level, or about $15,500. Eligibility levels won't change in the states that don't expand, but adults earning above the federal poverty line, or about $11,500, can receive federal subsidies in those states to purchase private insurance on the health insurance exchanges — so they still have a coverage option. This graphic from Kaiser helps illustrate the coverage gap in practice:

According to Kaiser's figures, Pennsylvania had one of the largest coverage gaps of any state that hadn't yet expanded Medicaid. It trailed only Texas (1.05 million), Florida (763,890), Georgia (409,350), and North Carolina (318,710). Texas Gov. Rick Perry isn't going to expand, and his likely successor also strongly opposes the expansion. Competitive governor races in Florida and Georgia this year could improve the prospects for expansion if Democratic candidates win. North Carolina Gov. Pat McCrory hasn't shown any signs that he's reconsidering his opposition to the expansion, as the state is involved in a pretty major overhaul of its existing Medicaid program.

There are other states, though, that appear to be on the cusp of following in Pennsylvania's footsteps.

The most likely candidate is Indiana, which has asked the federal government to approve an expansion plan levering an existing a state program using a health savings account model. If approved, that could eliminate a coverage gap of about 181,930 people, according to the Kaiser figures.

Tennessee Gov. Bill Haslam — who's been exploring Medicaid options for more than a year — told local reporters on Thursday that he could submit an expansion plan this fall. That could close a coverage gap affecting 161,650 people, but the Republican-controlled state legislature could oppose the effort.

Virginia Gov. Terry McAulifffe (D) also has a legislative headache on his hands. After the legislature rejected expansion plans this summer, McAuliffe offered vague overtures about exploring options to act unilaterally to expand coverage. Republican lawmakers have called for a special session in September to consider the expansion, which would close a coverage gap affecting 190,840 people.

Utah Gov. Gary Herbert (R) is trying to get an expansion deal done with the feds by the end of September, which would make 57,850 residents eligible for coverage. There are reportedly some sticking points in the negotiations with the Obama administration — particularly Herbert's proposal to attach work requirements to the expansion — but the state's Republican-controlled legislature could be Herbert's biggest obstacle.

And in Wyoming, Gov. Matt Mead (R) last week said he expects to offer potential Medicaid expansion plans to the legislature when it convenes in January. That would mean another 17,390 people no longer in the coverage gap.