Republicans are going to gain a lot of seats in the midterm elections. The big question is why. Political punditry has been saturated with arguments interpreting this result as a verdict of sorts on the Obama administration. Liberals are interpreting the incipient GOP win on poor communication or perhaps timid policies by the Democrats. Conservatives are interpreting it as the natural punishment for a party that moved too far left. (The mainstream media is taking up a softer version of this "center-right country" argument -- see the Hill's editorial arguing that "Democrats may have failed to learn lessons about overreach," an opinion which is noteworthy because that publication editorializes only when it can safely confirm the conventional wisdom.)

It's certainly legitimate to question the policies or the tactics of the Obama administration. But in order to have that conversation, you need to begin with a baseline expectation. What sort of performance should we expect normally? Clearly, in the current environment, it's not rational to expect the majority party to escape any losses whatsoever. If you want to blame the Democrats' loss on bad messaging or wimpy policies or rampaging socialism, then you need to establish how you'd expect them to do given normal messaging and policies.

Political scientist Douglass Hibbs has a model of the election. It takes account of three factors:

1. The presence of a midterm election, which generally results in losses for the president's party.

2. The incumbent party's "exposure" -- the more seats you hold, the deeper into hostile territory you're stretched, and the easier it is to lose seats.

3. Personal income growth, which heavily influences out-party behavior.

The model does not include presidential approval rating. Indeed, it doesn't include anything other than structural factors. That model predicts the Democrats will lose 45 seats in the House.

I've been emphasizing the role of structural factors like these in determining the midterm election. The point is not that structural factors determine everything, and that policies or communication or other tactical decisions have no impact. The point is to center the discussion around a realistic baseline. Political pundits generally ignore structural factors and interpret elections as either a contest of tactics or an ideological mandate. (Generally the former style prevails before the election, the latter style after, so that in October we read that one party's brilliant system for contacting voters in Ohio or a candidate's boneheaded gaffe will determine the outcome of the election, and afterward we read grandiose pronouncements about an electorate embracing the winner's political philosophy.)

In any case, without dismissing the post-election punditry altogether, it's worth keeping in mind beforehand a clear sense of what sort of result we would expect if the president's policies and political strategy made no difference at all. That's about a 45 seat loss. I suspect that figure is probably 5-10 seats too low, because President Obama rode a wave in 2008 that was unusually dependent on sporadic voters like the young and minorities, who tend not to turn out during midterm elections. He swept in a lot of House candidates who are going to have trouble winning a midterm election with a disproportionately old and white electorate.

Still, it's a rough ballpark baseline. If you want to have the "what did Obama do wrong" argument, you first need to establish what "wrong" would look like. That's probably a 50 seat-or-more loss.