Stephen Wolf at Daily Kos put together detailed analysis on the impact gerrymandering had on the House in 2012. In summary, the Democratic candidates won the popular vote, but due to gerrymandering Republicans came out with control of the House.
In November 2012, the American electorate voted to give Democrats unified control of government. President Obama won re-election with a majority of the vote, Senate Democrats increased their seat count, and Democratic candidates for the House won the popular vote. However, because of the way in which districts are drawn, Republicans easily maintained control of the House.
In particular, the lines in Republican-drawn states like North Carolina and Pennsylvania are grotesque. On the other hand, while Maryland is hideous, it’s only because Democrats drew it for parochial rather than partisan reasons. Democrats there easily could have drawn a much cleaner map where every district is safe.
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