I'm pretty convinced that there are critical upper bounds to population size and national surface area that guarantee that approaches that work great for small, tractable nations just cannot scale.
So step 0: Reduce the population of the US to the population of the metro Atlanta area (5.7 million in 2010).
Step 1: reduce the area of the US to the state of Maine (roughly).
Were states to implement such systems at the state level, I fully expect the smaller states to consolidate and form alliances of their welfare systems so as to achieve economies of scale faster.
My hypothetical was for them to socialize it with a shared single-payer system. Think about it this way: all of the small states in New England decide to create a government run insurance (like Denmark) by pooling all of their taxpayers.
An exchange would be more like a multi-payer system rather than a single-payer system.
So step 0: Reduce the population of the US to the population of the metro Atlanta area (5.7 million in 2010).
Step 1: reduce the area of the US to the state of Maine (roughly).