This contrast creates two powerful dynamics that will shape the unfolding debate. First, it means that with half the states inhibiting it, the program won’t enroll as many people nationally as Obama hoped. Because many of the resisting states have big uninsured populations and skimpy safety nets, 8 million of the 13 million people potentially eligible under the law’s Medicaid expansion live in states that are not increasing coverage, the Urban Institute calculates. Without state outreach, sign-up on those states’ exchanges also could slip, again depressing the national results.

That prospect frames a second, potentially decisive, question: Can the law’s supporters produce success in the states that are welcoming it? If California, Colorado, Minnesota, and other states that are implementing the law most enthusiastically can enroll large numbers of the uninsured, avoid big logistical snafus, and attract enough healthy young people to restrain premium rates, that would strengthen Obama’s hand nationally—and increase popular demands to participate in the resisting states. If it becomes “clear that in many of the states that are wholeheartedly implementing it, it is working well, the interesting question is, what kind of pressure will that put on the ‘refusenik’ states?” says Harvard University sociologist Theda Skocpol, who studies the safety net. In particular, success in participating states could inspire growing agitation from health care providers in the refusenik states, which are renouncing nearly $500 billion in federal money through 2022 (at small state cost) by rejecting the Medicaid expansion, the Urban Institute projects.