"One reason it’s difficult to read midterm elections as reflecting shifts in the “national mood” is that the actual set of voting people is quite different. According to exit polls, for example, the relative proportion of youth voters and senior voters shifted quite dramatically."
Well, but doesn't that also reflect the "national mood"? Two years ago, young voters were inspired and motivated to get to the polls. This year? Not so much. That made a difference in the results, yes. But their absence doesn't mean we're missing information on the national mood -- it's reflective of it.