In 2010, the newly passed Affordable Care Act—also known as Obamacare—mandated that states expand Medicaid eligibility to cover millions of uninsured Americans. But when the Supreme Court made Medicaid expansion optional for states, close to half opted out. Today, close to a quarter of states still have not increased access to Medicaid, and Medicaid expansion is now at the center of a national debate about President Biden’s Build Back Better legislation.