After conducting research that strongly suggested gerrymandering cost Democrats the US House in 2012 and after doing this same exercise for the Democrats, leading to a similar conclusion, I wondered how many seats Republicans might have left on the table that, with hindsight being 20/20, they could have won by making unilateral changes, within reason. Mostly it is through different, more aggressive gerrymanders, but partly it is from candidate recruitment and campaign execution in a few states that they could have won more seats. Let's take a look:
FL-18 - Recruitment
GA-12 - Recruitment
IN-07 - Redistricting
MA-06 - Campaigning
NH-01 - Redistricting
NC-07 - All of the Above
PA-17 - Redistricting
TN-05 - Redistricting
UT-04 - All of the Above
So that leaves us with 9 in total, leading me to believe the House could have been split 243 R 192D giving Republicans a net pick up of one. Below I'll go over the districts in detail case by case, but first a few that I left off:
AZ-09 - Did they really have a much better candidate? Sinema won by nearly 5%.
FL-26 - I think even an open seat would have gone Dem with Obama cruising here by 7 points and the district not being much worse downballot.
OH-03 - Drawing a 13-3 map technically is possible for 2012, but it puts a lot of Republican incumbents right on the threshold and their incumbents probably did balk, which was why Steve Austria was sacrificed.
WV-03 - With outside spending Rahall was basically evenly matched and still won by 8, I don't think more money would have made up for it.
WI-03 - While drawing a 6-2 map is possible, there's no way in hell that the Republican incumbents wouldn't have blocked it.
Now that that's out of the way, let's look at the districts in more detail.
FL-18
This one's a no-brainer. Allen West only lost by 1% despite representing about a quarter of the district and being a lightning-rod for controversial statements. The incumbent who represented the vast majority of the district, Tom Rooney, should have easily carried the district had he run here and I doubt we'd have gotten a candidate nearly as strong as now Rep. Patrick Murphy.
GA-12
This isn't as clear cut, but in a 44% Obama district Republicans should have definitely been able to defeat John Barrow. Their nominee Lee Anderson was underfunded and ran a totally inept campaign, had one of their other choices prevailed and gotten adequate assistance from outside groups, I think Barrow might have lost by 4 or so. With Anderson, it just made it too easy for Barrow to have the chance to define himself positively in a non-partisan manner while painting his opponent as less qualified, a strategy that works when one has convinced voters you aren't a partisan for the opposing team. Alternatively Republicans could have tinkered around the edges to gerrymander the district further, but the unknowable element there is at what black VAP threshold would the DoJ denined VRA preclearance? I'm of the opinion that the seat was a blatant Section 5 retrogression, but the DoJ obviously didn't agree with that, so the GOP probably could have gerrymandered it further.
IN-07
Here's our first one that's solely attributable to redistricting. Indiana Republicans drew a fairly modest gerrymander that if anything only made a difference in the 2nd district. The 9th was protected, but probably not to the extent that we'd have otherwise won it and the map as a whole looks quite clean and inoffensive. So what might they have drawn instead?
This map is a brutal 8-1 gerrymander that uses an NC-12 style district stretching from South bend to Gary to Indianapolis and ridiculously packs Democrats. The lines along the interstate could be cleaned up significantly with precinct splitting as they surely would have done. Here I've shored up the 2nd district a few points, the 9th a few points, and the 8th a crucial 4 points all the while turning the 7th district from Safe D to Safe R. Don't let that 48% Obama '08 fool you, he massively overperformed here and our US House candidates likely got blown out there in 2012. None of the 8 districts is really all that competitive for us and I'm strongly of the belief that this map would hold even in a wave year, or at worst see only one district or two at the very best fall.
MA-06
This district is a little more ambiguous and probably the most so of all I've included, but given John Tierney's narrow 1% win I believe that Richard Tisei's rather lazy campaign in the final stretch of the campaign cost him enough votes to lose. This was a district which was quite unfavorable to him on the face of it, but Tierney was scandal-damaged and thus quite vulnerable. It is also the least Democratic of all 9 Massachusetts districts, but Obama still won by a healthy 11%. Still, Tisei basically stopped running ads during the last leg of the race and political science research has shown that political ads have a very short half life of about a week or two at best beyond simple candidate introduction. As Tisei was never going to win this race by being personally popular himself, his not saturating the airwaves with anti-Tierney ads in the final weeks seems to have been his undoing.
NH-01
New Hampshire was drawn by Republicans to shore up Frank Guinta in the 1st district barely, but both he and neighboring incumbent Charlie Bass both lost by about 5% despite Guinta having a district 4 points redder. However this was totally avoidable and if I had been running the New Hampshire GOP, I'd have triaged Charlie Bass in a heartbeat. The guy barely squeaked out a 1.6% win in 2010 while Democrats got absolutely demolished in the state and posted atrocious approval ratings in subsequent polls early in the cycle. Given how Guinta seemed somewhat more secure and was a good deal to Bass' right, the GOP should have done what obviously makes sense in retrospect and just vote sink Bass' district.
This map does just that and gives Guinta a district that's roughly 4 points better than the one he lost and now Romney won it by 6%. Given how he needed to improve by just 2.5% in two party share, it stands to reason that Guina would win here. Even if he lost, Republicans would have a much easier time winning back the seat and doing so with a solid conservative than the will in the actually swingy NH-01 that Carol Shea-Porter holds. It doesn't look like they'll be winning back the 2nd anytime soon seeing as Obama won it with ease in 2012, so this is definitely a move that they should have done but thankfully for us did not. Finally, with a gerrymandered majority it makes sense for the GOP to do this as even though it lowers their ceiling of winnable seats, it does so for us as well which is all that matters.
NC-07
Given that this was the closest race in the country last year, any of the variety of factors would have made the difference here, but I think there's a good case to be made that the candidate was the most important. Republican nominee David Rouzer was from the wrong part of the district, hailing from the already staunchly Republican and much less conservadem Johnston County in the greater Raleigh area. Then you have the fact that McIntyre was able to bring the hammer down on him for being a lobbyist whose work resulted in jobs being shipped to India in a not so subtle bit of race baiting. Had Republicans nominated someone from the southeast of the district where all of the conservadems still live and vote, such as state senators Bill Rabon or Thom Goolsby despite their being freshmen, I believe they could have made up the less than a thousand votes difference that Rouzer lost by. Furthermore though, with such a small margin it's easily imaginable that Republicans could have either gerrymandered the district just a tiny bit more, or that Rouzer's campaign could have been technically improved enough to make up the 600-700 vote deficit.
PA-17
Here's our third state that's purely redistricting based and here it's that, despite Democrats winning the House popular vote in the state, they could have been held to a 14-4 deficit with a better map. Republicans in Pennsylvania, like those in North Carolina, already drew a terribly hideous map, but it could have been expanded upon like this:
Note that the southwestern districts are the exact same as the actual map, as well as 1, 2, 6, 7, and 16 as I don't see the need to tamper with them. Here though I've dismantled the PA-17 vote sink and replaced it with a very conservative district all the while maintaining the Republican safety in nearby districts. Tim Holden, who is uber popular in his Schuylkill County base, see's the county sliced and diced and the 17th being drawn worse for him than his old district ever was due to the decimation of his base. Even the neighboring 11th is a bad fit for him as it contains almost none of his old district. Now 17th district Rep Matt Cartwright has no good district to run in either. To do this I had unpacked the 3rd and 10th slightly and that was really it. Finally, given how 8th district incumbent Mike Fitzpatrick has announced he will retire in 2016 and if he follows through on this pledge, the 8th will be a tossup district. Here I've drawn it a crucial 3.5% more Republican downballot which would give Republicans a much better chance of holding it and this was done by splitting Bucks County, something that is not traditionally done but this is all a cherry on top of the 14th district.
TN-05
Tennessee saw the weakest Republican gerrymander of any state in that they didn't actually change the seat outcome at all over a non-partisan map and could have easily cracked the Nashville based 5th had their incumbents not complained.
Here I've done just that all the while keeping every Republican incumbent safe and happy with their home bases, or at least to the extent that the Republican controlled legislature already did. Districts 1-3 and 9 are all the same as in reality, but 4-8 are used to crack Nashville and the conservadem friendly parts of the rural Tennessee River Valley. None of the 4 districts containing Davidson County (Nashville) give Jim Cooper much of a chance as all are about as conservative as the state as a whole which Romney won very easily, as he would have done in all of these. Given Tennessee's Republican trend, I don't see us retaking anything even in a wave year, so Republicans really had little reason not to do something like this as every incumbent has a clear district to run in.
UT-04
Our final district is Utah's 4th where Jim Matheson was runner up to Mike McIntyre for narrowest Democratic win at roughly 800 votes. Unlike in North Carolina, redistricting was really the prime culprit as there was no excuse not to make his district just a touch more Republican. North Carolina Republicans at least had to balance out the adjacent districts; in Utah every non Matheson district was going to be overkill safe and it's not really worth posting a map given the narrow margin. Still, Mia Love wasn't exactly the greatest candidate as she, like Tisei, ran a lazy campaign in the end, even campaigning for Mitt Romney in Ohio rather than her district. Additionally, given the long history of racial discrimination in the LDS church, I wouldn't be shocked if there were a few hundred voters in the district who voted for Matheson rather than Love, who is black and that would make the difference. Finally, Love was a far right candidate and if she had been just a tad more moderate in just the campaign, she could have won. Republicans may yet win this seat anyway, but 2012 was really the best possible year for them to do so...
Honorable Mentions
Minnesota
Michele Bachmann nearly lost a freakin' R+10 district! That means Romney won there by a whopping fifteen points, yet she only won by one. Obviously it's not beyond the imagination that we might have pulled off the upset there and if we had Republicans would be hating themselves for renominating Bachmann. With her retirement, this clearly isn't a problem going forward, but it's still hilarious that we almost won a district this red for no reason other than that the Republican incumbent is just flat out crazy.
Nebraska
Republicans barely gerrymandered Nebraska at all, but that gerrymandering seems to have made the difference even if it was only a point or so. That's because Dems barely lost the 2nd district by a mere 1.6% with an underfunded nominee against playboy Lee Terry. You would think with Obama winning the 2nd district in 2008 and with Terry barely winning that year, they'd have made it more than just a point redder for safe measure, but no.
Here's a map they could have drawn, with the district getting significantly redder to the point that it is safe. This is done by having the 1st dump the bluer half of Lincoln in the dark red 3rd, allowing it to take in some very blue parts of Omaha. All three districts are safely red and I doubt we'd stand a chance in them even in a wave, whereas under the actual map we could win the 2nd in a presidential cycle.
So in conclusion, there were a likely 9 seats that Republicans essentially gifted to the Democrats that I believe they could have won when considering these three criteria to a reasonable degree, which is unsurprisingly far lower than the 20 or so that I suggested Democrats might have forfeited and that is mostly because Republicans very aggressively gerrymandered the states they drew while Democrats to a large degree did not.