John Sides and Eric McGee take up the question of whether Republican gerrymandering cost Democrats the House in 2012. It's a complicated post, but I'll attach the muddled conclusions and let you decide. One thing is certain: gerrymandering, while certainly real and effective, mattered less in 2012 than you'd think:
The analysis above does not confirm the worst fears about the “great gerrymander” of 2012. But given the challenge of answering “compared to what?”, we would not argue that the 2011 redistricting gave the GOP no advantage whatsoever. Political science research on redistricting has confirmed that control of the line-drawing process does yield some benefits. The challenge is in estimating what those benefits are. We have tried to show that the answer is far more complicated, and that the magnitude of the redistricting effect is probably smaller than many have assumed.
Moreover, our argument should not be construed as a defense of how redistricting is currently done. Reforming redistricting by handing responsibility to independent commissions might produce more even-handed results. In fact, the evidence from California suggests the state’s new redistricting commission produced a fair plan that met the mandate of the law. We’d support using these commissions in more states.
Plus, the need for such commissions is arguably increasing. Gerrymanders will likely be more effective in the future because the partisanship of a district, independent of incumbency or any other factors, is a more important predictor of House elections than it was several decades ago. That makes redistricting a more powerful tool than before.