First watch this video.
It is from the 2012 Republican Primary Debate in Iowa.
Look at that standing O at the end when they all raised their hands. That is the Republican Primary voters that a Republican congressional candidate fears. They fear a primary challenge on the right more then anything else and rightly so.
Number two is redistricting that happened as the result of the 2010 Census. The Republicans won big in 2010 with many gubernatorial victories and the control of several state legislatures. They re-wrote the congressional districts in their favor and to be fair the Democrats would have done the same.
Number three is just how gerrymandered the current congressional districts are. From @redistrict’s twitter stream is the grand total of all the House races. It looks like the final results are all in now and the Democrats won a total of 1,362,351 more votes then the Republicans or about 1.12 percent more votes. The Republicans though control 234 seats in the house compared to 201 Democratic seats a margin of 6.6 percentage points in the Republicans favor. Now because of the way the house is set up as a representational body the vote percentages do not typically equal the seat percentages. Usually when their is a blow out by one party or the other then that blowout is exaggerated in the house. When the election is relatively close, and 1.12 percent is close historically, then the distribution of the seats is close plus or minus a little wiggle room.
This is not the case at all in 2012 and that is because of how badly gerrymander the districts are. The Republicans should have more seats then the democrats but not at the margins they hold now. The way districts are proportioned for the House favor a party with a strong rural base. The democrats run up big margins in their urban districts but that still only lets them when that one district. Those facts will always favor the rural party which right now is the Republicans but it should not favor them at nearly SIX FOLD their vote percentages. The gerrymandering does that.
Also just as an aside every state has now certified their results but Hawaii so it looks like the final vote total in the presidentidal race are 65,899,557 (51.06%) of the votes for Obama and 60,931,959 (47.21%) for Romney giving Obama a 3.85 percent margin of Victory. Not the blowout that 2008 was but almost 1.5 points better then the Margin of Bush over Kerry in 2004.