It surely played some role, but if it was significant, we would have seen more gains as a result. I'm not sure many Republicans took now-Democratic districts, and the idea that the 2010 lines with 2012 votes would have equaled a Democratic takeover misses a lot of points regarding blue states losing seats due to migration and such.

It can't be discounted, but if there's truly a leftward shift, it would actually show in the data. Vote shares don't show it, and Obama having longer coattails doesn't show it, either.