1. A President’s party makes big gains when he’s elected, but rarely when he’s re-elected. Even in a landslide Presidential victory.
2. While Republicans control a disproportionate share of toss-up districts, Democrats need to increase their share of toss-ups from 32% to 60% to gain the majority. That doesn’t happen in a re-elect.
3. Republicans control redistricting in a lot of states. While they may not gain seats in those states, they are unlikely to lose many. A lot of toss-up districts will be safer.Democrats can really only redistrict Republicans out in Illinois.
4. There were another dozen pick-ups in heavily Republican districts where it was a fluke for a Democrat to win. Republicans didn’t win those types of districts in 2010. While Republicans do control 61 districts Obama won, John Kerry won only 13 of those. Obama took more than 58% of the vote in 1 district and took 57%+ of the vote in four districts. John Kerry got 53% in his best district. Republicans don’t hold marginal districts. So they can't lose them.
5. While Republicans have only 1 district where Obama exceeded his vote by 5 points, Democrats have 9 districts that exceeded McCain's vote by 5 points.Thus, the Republicans have better opportunities that will likely only get better with redistricting. Two of those districts are in Arkansas and West Virginia, states where Obama may make things worse.
6. Republicans won 13 southern House seats in districts that have always elected a Democrat, despite voting for Republicans for the senate and President. When the Democrats lose a seat in the south, they don’t win it back.
7. Democrats had a huge advantage due to Republican scandals. In some cases, they took scandal ridden seats, but they effected others.
8. Republicans only hold one legislative body. A party almost always generates the type of animosity that results in big losses when they hold the Presidency.
9. Democrats were only able to take the House and Senate in years when Republican turn-out was low. That’s unlikely in 2012.