The reason is that the way the law was designed, the federal government only agreed to cover the full cost of the expansion until 2016. Starting in 2017, states will have to start kicking in money, and by 2020, will be expected to cover 10 percent of the cost of the expansion.

According to the Congressional Budget Office, state spending on Medicaid will be $46 billion higher in the coming decade than it would have been before Obamacare passed (which is on top of the $792 billion cost to federal taxpayers). The Center on Budget and Policy Priorities has noted that this still represents a mere 1.6 percent increase over the $2.9 trillion that states would be spending on the existing Medicaid program anyway.

But this understates the cost of the Medicaid expansion in several ways. To start, the $46 billion figure is for 2015 through 2024, meaning that for half of the period being measured (2015 through 2019), the state share of Medicaid won’t have reached its full level. Additionally, the CBO analysis is being made at a time at which two dozen states have not agreed to expand Medicaid. If every state were to expand Medicaid, the total cost to the states would be higher than $46 billion. This also assumes the federal government keeps paying the same share in perpetuity — which is far from assured.