In a word, the economy. People tend to have short memories when the economy is bad. All they know is; the economy’s bad. It must have something to do with the way government is handling it regardless of which party is in favour. Throw the bums out.
I am still doubtful that we’ll see a repeat of 1994 when the Republicans picked up a majority during the Clinton administration. The difference between then and now is all the negative publicity the GOP is generating; various personal indiscretions, the Tea Party, and the fact that it has become readily apparent that they actually have no governing ideas to offer. Yet the American people seem poised to give them back control of the country.
There must be some Republican ideals that have a broad enough appeal to bring the undecided and/or unenthusiastic to the polls. That and they can always count on the Democrats to shoot themselves in the foot.