Nearly two weeks have passed since the Supreme Court issued its ruling on the constitutionality of President Obama’s signature health care reform law, the Affordable Care Act (ACA). Although the mandate directing “individuals to purchase insurance” remains intact, the provision that would have required each state to provide Medicaid coverage to all working adults with incomes at or below 133 percent of the federal poverty level (FPL) was axed. Had the Medicaid stipulation passed – arguably the greatest virtue of the ACA – then every family of four earning less than roughly $31,000 would have been covered. This would have resulted in coverage for over 16 million new enrollees, or almost half of all those newly insured under the ACA.

Medicaid was originally enacted in 1965 to enable states to provide medical assistance, as well as rehabilitative and other services, for certain families and individuals whose income and resources are insufficient to meet the costs of medically necessary services. The program has evolved over time and today constitutes the nation’s primary health insurance program for low-income and high-need families and individuals.

Now that Medicaid expansion is non-compulsory, however, there has been great speculation as to how many currently uninsured individuals will ultimately gain coverage under the law. At the time of this writing, only 13 states plus Washington D.C. have agreed – some formally, some informally – to participate in a program for which the Congressional Budget Office (CBO) estimates the federal government will bear 93 percent of the tab over the first nine years of implementation. In total, 37 states have either opted out of the program or are currently undecided.

Although the CBO analysis demonstrates that between 2014 and 2022, the ACA’s Medicaid expansion will add just 2.8 percent to what states spend on Medicaid (most of which will be offset by reducing costs for uncompensated care), a number of GOP governors have derided the program claiming that they can’t afford even a marginal increase in expenditures despite the likelihood of dramatic increases in coverage.

Here’s how ACA Medicaid works

If a state opts in, then all adults with incomes at or below 133 percent of the FPL will receive Medicaid coverage.

If a state opts out, then those without who currently don’t meet state-specified eligibility requirements for Medicaid will not receive it. Those who are earning less than 100 percent of the FPL and not receiving Medicaid benefits will not be eligible, but those earning between 100-133 percent of the FPL will likely qualify for subsidies to buy a private plan. Here’s the problem: Eligibility requirements and limits for Medicaid vary by state; therefore, many uninsured individuals and families not already covered under a state’s current Medicaid rules could be left without any options for coverage if their state rejects the expansion.

I have compiled data to explain coverage ceilings, eligibility limits and participation by state. Have a look at your state.

As of this writing, nearly 8.2 million Americans fall within this pernicious “no coverage” zone. Stated differently, if every state decides to participate in the program, then 8.2 million Americans would gain health insurance under Medicaid. Somewhat ironically, it’s those states that offer the most generous Medicaid eligibility limits and already cover most of their population that have committed to the federal partnership. For instance, of the 13 states plus Washington D.C. that have already made a commitment to participate, 11 already cover parents with children at or below 100 percent of the FPL. And of the six states that make Medicaid available to virtually all childless adults, five have already opted in.

The non-obligatory nature of extended Medicaid participation for states creates an incentive structure that is both retrograde and counterintuitive. It is precisely the states that could most benefit from the increased coverage under ACA’s Medicaid expansion provision that seem least willing to participate in the plan.