Republicans used the 2010 census to gerrymander districts in their favor.
Republican strategists spent years developing a plan to take advantage of the 2010 census, first by winning state legislatures and then redrawing House districts to tilt the playing field in their favor.
Their success was unprecedented.
In states like Ohio, Michigan and North Carolina, Republicans were able to shape congressional maps to pack as many Democratic voters as possible into the fewest House districts.
The practice is called gerrymandering, and it left fertile ground elsewhere in each state to spread Republican voters among more districts, increasing the GOP’s chances of winning more seats.
http://www.pbs.org/newshour/rundown/gop ... ems-house/
Because of gerrymandering they were able to keep control of the House in 2012 even though Democrats actually got more votes.