States have a lot of discretion in implementing the Affordable Care Act (ACA), and some are using that leeway to undermine the expansion of health insurance coverage. In a new blog post, The Commonwealth Fund's David Blumenthal, M.D., and David Squires point out that "nonparticipating states have enjoyed only limited success in preventing the ACA from taking root in their jurisdictions." More than 3 million individuals have enrolled in a health plan through the marketplaces in nonparticipating states, and Medicaid enrollment rose in half of the nonparticipating states because of the so-called "woodwork effect."