Legislators in Arkansas, New Hampshire, South Dakota and elsewhere have spent the past three months considering whether they should expand Medicaid under the terms of the Affordable Care Act, or renew their previous commitment to do so. But new projections by the Congressional Budget Office (CBO) and Joint Committee on Taxation (JCT) indicate the federal government’s cost of Medicaid expansion over the next decade will be significantly higher than originally expected. This raises serious questions about the program’s financial viability.

Under...