Medicaid historically had been reserved for only the poorest and sickest, but the ACA opened it up to the lower middle class. States that expanded the program have experienced many benefits. For example, uninsured rates dropped -- dramatically in some states -- as did uncompensated care.

"Medicaid is such a fabric on the health system. Now, there are very few -- if any -- policy reasons not to expand," says Adam Searing, associate professor at Georgetown University's Center for Children and Families.

Medicaid expansion has been a highly partisan issue. But the debate at the federal level has revealed that there's more bipartisan support -- among voters and policymakers -- for expanding Medicaid than previously thought. Republican governors arguably scored the biggest win with the demise of Paul Ryan's plan because now they will likely take less political heat for expanding Medicaid and can claim credit for insuring more of their residents.

With the ACA here to stay for the foreseeable future, Searing notes four states worth keeping an eye on: Florida, Georgia, North Carolina and Texas. Once one of those states expand Medicaid, "then the dam breaks," he says.