The big question is whether the Republicans will retain the House in 2012. The answer is almost certainly yes. Remember that I said it was definitely flipping this year and that the Senate wouldn't. Here's by the numbers:
Redistricting net impact: No net seat change
McCain PVI+: Republicans 228 Democrats 207
Bush 04 PVI+: Republicans 237 Democrats 198
Era Average: Republicans 227 Democrats 208
Obama wins Re-election: Republicans 220 Democrats 215
Obama loses Re-election: Republicans 237 Democrats 198
I don't see redistricting helping either party too much. The Republicans control a number of redistricting plans, but they have so many seats in those states that any redistricting will just make sure they don't lose seats. Republicans have a 19-6 advantage in Florida. The state will gain one or two seats. No one knows how Fair Districts Florida will impact the districts but it's safe to say that the GOP will have a difficult time retaining such an advantage. Even in a good year I could see Republicans ending up 17-10.
Texas is similar. The GOP has a 23-9 advantage. They'll have to help their most vulnerable members. To do so, they'll create a map that'll likely result in a 25-11 or 24-12 GOP lead. Despite control of the redistricting process in Michigan, Ohio, and Pennsylvania, Republicans are so overextended that they're likely to lose two seats in those three states. Republicans simply have too many seats concentrated in certain states to benefit even if they're drawing the lines.
McCain and Bush PVI are the number of districts that Republicans did better than +2.5 points with Bush and -6.4 points with McCain. Since 1994 the Republicans have averaged 227 seats.
People talk a lot about coattails. They aren't as strong in a re-election.
Since World War II new Presidents have averaged about a +16.6 from the expected number of seats. Obama's coattails were longer than anybody else has had. That could indicate he'll bust out of the expected number in re-election.
The coattails disappear in mid-terms. While it's been depicted that a President gets clobbered in mid-terms it's really just him losing the seats he won when he was elected. A return to normal. Not only did Obama do better when he was elected but he was the most below expected in mid-terms. So maybe the Obama magic isn't ever lasting.
When a President runs for re-election his coattails aren't nearly as long. If he wins his party ends up around 6.4 seats over expected. Reagan, Nixon, and Eisenhower all won re-election in landslides, doing significantly better than they did win originally elected. Their congressional totals averaged 8 above expected, compared to the 19 they had in the original election.
I'm almost certain the Democrats will gain seats in 2012. In worst case scenario above they're +5. In best case, they're +22. It's conceivable Obama is on the high end. If he equals Nixon the Democrats take the majority. Of course he also had the worst mid-terms too.