Like most people, I like to get to get a lot of mileage on one tank of gas. When it comes to predicting Congressional elections, this means I try to get as much as I can out the national generic ballot question. Yet, it is useful to know the limits of this tool.
1. Like any poll, be aware of the sample population. In the past few weeks, Rasmussen (likely voters) has shown Republican leads of 3-6%, while Public Policy Polling (registered voters) shows the same lead for the Democrats. While the difference between the two pollsters may or may not fully explained by sample universe, it definitely is part of the difference. Not surprisingly, all other pollsters employing a likely voter model have also found that over the past few months the Republicans lead on the generic ballot by anywhere from 2-7%. 2. Democrats are going to need to do better than a tie in the national vote to earn a tie in the seat count. Due to incumbency advantage (among other factors), the party in control of Congress (the Republicans) can actually lose the national House vote and still maintain a majority. In 1996, for example, Democrats slightly bested Republicans in the national House vote, but were only able to gain 2 seats off their 1994 performance. 3. Re-districting efforts make converting vote to seats harder. If you look around the web trying to find a vote to seats curve you'll quickly notice that many of them leave out re-districting years (e.g. what 2012 is). Any estimate of vote to seats will be difficult in 2012 without a re-adjustement for re-districting, and we still do not know how many of the state Congressional maps will look. 4. Despite difficulties in estimating a vote-seat curve, it seems reasonable to believe that the 2010's re-districting will be not be pretty for Democrats. Republicans hold control of re-districting efforts in more states than usual. Therefore, Republicans could potentially do better than any prior vote-curve suggests.
5. A model that a friend of mine (who has specifically asked for no credit) helped develop illustrates how favorable re-distrcting in the 2000's (not anywhere as favorable for Republicans as the 2010's) definitely helped Republicans.
Utilizing just prior seat percentage in Congress controlled by Republicans, national vote, and a dummy for the 2000's and 1950's, we get a rough guide (95% confidence interval of +/- 14 seats) for how many seats Republicans gained in Congressional elections (including re-districting years) since 1942. Note, the model would have out-of-dataset predicted Republicans controlling 240 seats after the 2010 election.
6. If we assume that re-districting efforts of the 2010 census yield a situation no worse for Democrats than the 2000's re-districting, they need about 51% of the two-party vote for Congress to have a 50/50 chance of gaining control. If Republicans win the national vote by 2% (as suggested by their smallest lead on the likely voter models), they win about 232 seats. Note Republicans will likely do slightly better than these conversions suggest.
So what does all of this stuff mean? Be careful of reading too much into the generic ballot this year, but also that Republicans will likely do better in seat count than the national vote suggests.