Brain Beutler looks at how Romney could undermine his own law:

Unlike the ACA, the Massachusetts law has two separate markets — one for people living under 300 percent of the poverty level and thus qualify for insurance subsidies; one for people above that threshold. The subsidized pool is called Commonwealth Care. For that market to work, Massachusetts relies on the federal government, via Medicaid, to cover half the cost of the generous subsidies it provides to lower income individuals. If Romney were to block grant Medicaid and cut its spending as dramatically as he’s signaled he would, Massachusetts would slowly lose those dollars.