But a CBS News report discusses a growing source of disquiet: In almost half the states with exchanges, the overwhelming majority of enrollments are coming from Medicaid, not the new insurance markets — 87 percent in Washington, 82 percent in Kentucky and, last time I looked, 100 percent in Oregon (which delayed opening its insurance exchange in order to work out technical bugs). The Medicaid expansion side of the bill seems to be working fine in the states that opted for the expansion. But the private insurance side doesn’t seem to be getting a lot of pickup.

That’s a problem for three reasons. First, signing up for Medicaid is a comparatively simple process, which means that we don’t really know how well things are going on the private side in many of these states. Second, insurance products need a pretty big pool of customers in order to be stable; otherwise, there’s too big a risk that you’ll have a wildly disproportionate number of sick people. Obamacare has risk-adjustment mechanisms to try to mitigate this problem, which I discussed the other day, but they only defray some of the expenses for an insurer that gets too many sick people. Besides, the mechanisms are only temporary; they go away after 2016.