As Nixon noted, the federal government will initially cover the entire cost of the expansion in states that choose to participate. The federal share begins to fall gradually in 2017, bottoming out at 90 percent in 2020.
The Obama administration has emphasized the generous federal payments in pushing states to accept the Medicaid expansion, and it has said states that opt in now can back out once they become responsible for more of the expansion's costs.
Until the Supreme Court made the Medicaid expansion voluntary, it was set to cover about half of the roughly 30 million uninsured people set to gain coverage under the Affordable Care Act. That share will fall as states opt out of the program, so each state that signs on is a boost to the law's ultimate chance of success.
— This post was updated at 12:18 p.m.