according to CBS channel 2 many states have made changes in their districts which make it easier for incumbents to raise money and difficult for challengers.
"the incumbent protection system" -CBS
the end result is, we had little change in the House and the Senate because of this.
basically, IMO, the Democrats changed the rules to protect themselves from a major shift due to passing the illegal Health Care bill.

a couple of years ago, the state of Nevada changed their districts which gave power to the South (democrats) and took it away from the North (Republicans), so the end result is, Nevada leans Democrat now.

believe it or not, California is about 40% Republican, which is a huge number, but the Republicans never question the winner-take-all system in the state.

this is how we get to the Democrats holding a large number of offices, with a minority.