It shouldn't be hard to divy up states (granted you might have to cut a state into more than two parts in some cases) in a way that results in no net gain for either party. It's a simple math problem.
Yes, you could slice Oregon in half horizontally and maintain the same number of reps, but then you'd have two new states with the East feeling divided. If you split it vertically, then you have the problem of uneven representation.