Actually, it made a big difference in the Senate. The Republicans were well placed a year ago to take control of the Senate, but instead they lost seats. Most startling was Scott Brown, who got thrown out despite having broad public approval from his constituents. He was tossed out because he was a Republican, not because he did a bad job.
In any case, the Senate is a very different creature from the House due to the nature of the constituents they represent. Senators represent the entire state and thus the entire state votes for them. As such, there is no impact from redistricting. House seats, on the other hand, are manipulated very heavily by the states (red and blue alike) to ensure that the party in-power in that state keeps as many of their state's representatives as possible. This results in very odd districts that tend to be resistant to any small or moderate demographic or political changes. Large changes in House representation require truly significant shifts in public sentiment or demographics, and that certainly wasn't present between 2010 and 2012. Minor changes will usually return a relatively static House.
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