You can pack a lot of your opponents into a few districts, but if your own districts are only 55% for your own party, a 10-point swing can knock you out of office. And the swing from 2010 to 2012 is currently about 9 points. Another answer is that redistricting is sometimes done to protect specific incumbents – which results in packing one’s own party members into a district. On average, the whole thing could well be a wash.
Charlie Cook remains skeptical:
In the House, we have not yet seen any signs of deterioration for the GOP majority. Even if Democrats were to win every seat currently rated solid Democratic, likely Democratic, or lean Democratic, as well as every toss-up, they would still come up short of a majority. The canaries in the coal mine are GOP seats currently rated as lean Republican or likely Republican. Cook Political Report House Editor David Wasserman points out that with Democrats likely to lose perhaps 10 of their own seats, they would have to gross 35 seats to hit the 25 net seats necessary to win a majority. That’s a very tall order.