http://www.commonwealthfund.org/publications/blog/2017/feb/states-roll-back-aca-medicaid-expansion

Millions of people have gained health insurance coverage though the Affordable Care Act’s (ACA) Medicaid eligibility expansion, adopted by 31 states and Washington, D.C., over the past three years. Should Congress decide to eliminate or reduce federal funding for this coverage as part of ACA repeal, states that expanded will be faced with the prospect of either maintaining coverage out of their own funds or dropping the new beneficiaries from the program. Along with the loss of coverage or the creation of large budget holes, rolling back Medicaid benefits would present states with expensive and complex administrative challenges.

Were Congress to repeal federal funding for the expansion group, coverage for these newly eligible enrollees—estimated at 11 million as of 2015—inevitably would disappear. No state is in a position to support this population without considerable federal funding. For example, were ACA Medicaid expansion funding to disappear, California would lose more than 27 percent of the total federal Medicaid funding the state is projected to receive over the 2019 to 2028 time period; federal funding would drop from $364 billion to $265 billion, a $99 billion loss.

If a repeal bill retains the Medicaid expansion but reduces federal funding for the expansion group to traditional Medicaid funding levels,¹  some states might seek flexibility to roll back coverage to a lower level such as 75 percent of poverty, rather than the eligibility standard used under the expansion (138 percent of poverty). Rather than terminate insurance eligibility altogether for populations without an alternative source of coverage, states also might try to trim benefits or reduce or freeze provider payments.

But even if funding is eliminated completely, federal laws place important brakes on the process. For example, federal Medicaid rules dating back decades require states to determine if there is another basis of eligibility prior to terminating coverage. At least some of the people covered as part of the expansion population may qualify for Medicaid on other grounds such as pregnancy, being the parent of a minor child, or disability.