The Avalanche!

Ellisblog!: 10/17/2010 - 10/24/2010:

Mid-term elections are, generally speaking, a zero sum game. The out-of-power voters are highly motivated to vote, to express their exasperation with the in-crowd and send a message. The in-party's voters are disheartened and discouraged and therefore less likely to vote. Three percent more of the former and three percent less of the latter adds up to a six percent swing and some number of Congressional seats (usually around 20) changes hands."