Last week, I cited a model showing that based solely on the number of seats held by the Democrats going into a midterm and personal income growth, Democrats would be expected to lose 45 seats in the House. That's based solely on fundamentals, with no consideration of the president's approval rating or anything else about how the public views his priorities. I did say I thought that total was 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."

The exit polls from today's election show how this happened. The non-white share of the electorate fell from 24% in 2008 to 19% in 2010. But the age gap is the real tidal shift. In 2008, Republicans won voters over 65 years old by 8 points, but were crushed among voters under 30 by more than 30 points. The under 30 vote outnumbered the over 65 vote.

In 2010, Democrats still crushed Republicans among the under 30 vote, albeit by just 20 points. But the over 65 vote went Republican by a massive 20 point margin. What's more, in today's election, senior citizens constituted more than twice as high a share of the electorate compared to voters under 30. In 2008, the young were 18% of the electorate, and the old were 16% of the electorate. In 2010, the young were 10% of the electorate, and the old were 24% of the electorate.

A huge portion of what happened in this election, then, reflects the fact that Obama won with a coalition unusually dependent upon the young, and that he failed to turn these sporadic voters into regular voters. Democrats' strength among the young bodes well for them in the long run, but as Keynes famously said, in the long run we're all dead. In the short term, Republicans are back in command of the House. And Democrats will probably continue to struggle disproportionately in midterm elections.