Why are young voters so unenthusiastic about voting? Not because they're disillusioned. Because they're satisfied with how things are going and feel little urgency. Here's Political Wire:

The latest NBC/WSJ poll found that young people much less enthusiastic about voting in the midterm elections than older people. Just 35% of those 18-34 years-olds are enthusiastic about the midterm elections, versus 65% of seniors.

A Congressional Connection poll may have found the reason: When asked if they were satisfied "with the way things are going in this country today," 54% of 18-29 year-olds said "yes," compared with just 28% of 30-49 year-olds, 22% of 50-64 year-olds, and 19% of 65+ year-olds.

Meanwhile, Brendan Nyhan suggests that, contra my speculation, Democrats aren't necessarily less able than Republicans to sustain their enthusiasm. (Though he is working off a very limited data set.)