As D-Day looms for ObamaCare, one big question is how many states will sign up for its Medicaid expansion. The recent and spectacular flip-flop of Arizona Governor Jan Brewer is a case study in the political pressure and fiscal gimmicks designed to get states to succumb. It's also a study in the arcane and perverse ObamaCare incentives that are intended to gather ever more health-care spending under federal control.

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Arizona's current Medicaid program is well run by the program's standards—a low bar—but it is also too large....