I think that path would look something like this: With health-care reform either repealed or overturned, both Democrats and Republicans shy away from proposing any big changes to the health-care system for the next decade or so. But with continued increases in the cost of health insurance and a steady erosion in employer-based coverage, Democrats begin dipping their toes in the water with a strategy based around incremental expansions of Medicare, Medicaid, and the Children’s Health Insurance Program. They move these policies through budget reconciliation, where they can be passed with 51 votes in the Senate, and, over time, this leads to more and more Americans being covered through public insurance. Eventually, we end up with something close to a single-payer system, as a majority of Americans — and particularly a majority of Americans who have significant health risks — are covered by the government.
One question is whether having both Medicaid and Medicare (and other programs) function as a “single payer” system, but that is arguably semantics. In any case the American system is likely to remain fragmented. I am also not sure if this process would take a decade, as sometimes a single election cycle can feel like an eternity. In any case, I see that as likely a superior outcome to the current ACA track. I have never thought that a mandate is workable in a fragmented system with employer-based care and high health care costs and high income inequality.
I also would not be surprised to see Romney, if elected, and if ACA is struck down, resurrect some version of the McCain health care plan with tax credits, maybe some more federalism, and less of a Medicaid extension than was in ACA. I don’t know if that would pass but I suppose I think not. I also don’t see much hope for a much-needed “supply-side” competitiveness plan.