What states will participate in Obamacare’s expansion of the Medicaid program? The answer to that will have a profound effect on national and state health-care budgets and the number of uninsured US residents for years to come.

The president’s signature Affordable Care Act raises the income level for Medicaid eligibility up to 133 percent of the federal poverty line. That’s one of the major ways the health reforms attempts to expand coverage. If all states participate, 21 million will get Medicaid coverage, reducing the number of uninsured in America by 48 percent, according to a Kaiser Family Foundation analysis.

But the Supreme Court ruling which upheld ACA’s core individual mandate for health insurance also held that Uncle Sam can’t make states go along with the Medicaid expansion. So right now, governors and state legislators across the country are facing a tricky decision: Should they dive in and accept the expansion? After all, the federal government is promising to pay virtually all the costs of such a move, at least for now.

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