A provision in the ACA called for expanding Medicaid eligibility in order to cover more low-income people. But in June 2012, the Supreme Court ruled that states could not be forced to expand their Medicaid programs, so it was left to each state to determine whether to participate or not. As of 2017, Medicaid had been expanded in 31 states and the District of Columbia. Maine was poised to become the next state to expand Medicaid after voters passed a ballot initiative calling for Medicaid expansion in the state (Maine lawmakers had previously passed Medicaid expansion bills five times, but the Governor had vetoed all five of them).
The federal government is financing most of the cost of expanding Medicaid, and a small portion is being paid by participating states. The costs for enrollees who are newly eligible under the expanded guidelines was covered 100 percent by the federal government until 2016. That will gradually decrease to 90 percent by 2020, but will not go below that amount.
Nearly 10 million people have gained coverage as a result of Medicaid expansion. However, in the states that have not expanded Medicaid, there’s a coverage gap that leaves about 2.4 million people ineligible for any sort of affordable coverage. To be clear, this is due to the decisions that their states have made, rather than a flaw in the ACA itself. The states could opt to accept federal funding to expand Medicaid at any time, thus closing the coverage gap.