I have done some looking at gerrymandering and how it cost the Democratic party the US House in this year's election. This isn't deep research, but rather just comparing raw votes with representation. The disaster that was the 2010 midterm elections is still wreaking havoc on Democrats. Not only did Republicans regain the House, they took over many statehouses and also many state legislatures. Nowhere is this damage more evident in purple states. More below .
To keep this short and sweet, I am assuming Republicans will end up with a 234-201 advantage in the House. I looked at only swing states won by Obama in both '08 and '12. Recall that Michigan, Wisconsin and Pennsylvania haven't been carried by a Republican since the last century and that Obama has won these states twice, yet Republicans will outrepresent Democrats 68-33!
A far more representative 51-50 Democratic edge would have put the Speaker's gavel in Nancy Pelosi's hands. Even a 50-51 minority in these purple but Democratic-leaning states would have done the trick. This is giving the red states, including Texas, their due to redistrict their states in exchange for Democrats keeping their advantages in New York, California, Illinois, Massachusetts and Maryland. All that is needed is fairness. I've said this before--fairness is good news for Democrats.
The good news--Democrats don't have to win all these state legislatures by 2020. They need to take away trifectas so that fair lines can be drawn. Obviously, all of the governorships will be up for election more than once. Scott, Kasich, and Snyder are unpopular figures in their states. While Walker was not recalled, the swing votes in that election were people that didn't believe in recall, not whether Walker was doing a good job. Corbett in Pennsylvania can't be too secure in a very much blue leaning state. Virginia's gov is term limited. Democrats can win any and all of the gubernatorial races.
Here's the partisan breakdown for the next Congress in the purplish state won by Obama (all Republican majorities):
Colorado 4R 3D
Virginia 8R 3D
Wisconsin 5R 3D
Florida 17R 10D
Michigan 9R 5D
Ohio 12R 4D
Pennsylvania 13R 5D
Total 68R 33D