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.
As the polls and expert analysis for Senate races in 2012 begin to come in fast and furious, one question we all should be asking is "do the polls and analysis actually mean anything?" To help answer this question, let us first look at polling data from early on in the last Presidential year Senate campaign (2008). As we will see, one should proceed with caution when examining Senate polling data this early in the cycle. In 2008, Democrats gained 8 Senate seats from Republicans. By the end of 2007, only 12 eventual match-ups had been polled (according to Pollster.com). I have taken an average of all the polls for 2007 in these races. I have converted the Democratic and Republican percentages to two-way (i.e. allocated undecided and third party votes to the two major candidates proportional to their initial unaltered percentage). Of the 12 match-ups polled, the average error between the early polls and actual margin was 10.9%, while the median error was 8.3%. That is about double the average pollster error seen in Senate polls in the final 21 days of the campaign from 1998-2008.
More interesting is the direction of the error. In the Democratic year that 2008 became, Democratic candidates over-performed their early projected total by an average of a little over 4.1%.
Of the 12 races polled in 2007, Democrats over-performed in 9 of them. Of the 9 over-performances, Democrats actually won 3 (Minnesota, North Carolina, and Oregon) races they were polled to lose. The early polling correctly projected that they would pick up 4 (Colorado, New Hampshire, New Mexico, and Virginia) other seats. The 8th seat pick-up, Alaska, was not polled this early in the cycle.
What about non-partisan analysis? Non-partisan analysis this early in the cycle in 2006 and 2008 (2010) under-predicted Democratic (Republican) gains.
At this point in 2006, the invaluable Cook Political Report placed 9 previously held Democratic seats in the likely, lean, or toss-up column, while only placing 6 Republican seats in these designations. Democrats would go on to gain 6 seats from Republicans. 5 (Missouri, Ohio, Pennsylvania, Rhode Island and Virginia) of those 6 were in either likely, lean, or toss-up column at this point. Only Montana proved to be a surprise. In 2008, Cook had 5 Democratic seats and 6 Republican seats in three aforementioned columns. Of the 8 seats Democrats eventually gained, only Colorado, Minnesota, New Hampshire, and North Carolina were listed in one of the three possibly competitive categories. 2010, similarly, broke differently than early analysis pegged. Cook put 8 Democratic seats in the three competitive categories, while putting 9 (including later party switcher Arlen Specter and 2010 loser) Republican seats. Republicans would take 6 seats in the 2010 elections. 3 (North Dakota, Illinois, and Wisconsin) of these were listed as competitive at this point in 2010, while 2 (Arkansas and Indiana) were solidly Democratic.
What does this mean for 2012?
First, as the Cook Political Report figures suggest, there are going to be some seats that may not be viewed as competitive at this point that could become competitive and vice-versa. Some of these newly competitive seats may be due to retirement (see Indiana 2010), while others may just be campaigns evolving over time (see Oregon 2008).
Second, very early polling and analysis under-forecasted the waves that would occur in the following year. Intrade (and my own preliminary Senate model) indicate that Republicans are the ones who will benefit from any 2012 wave. Right now, Cook has an amazing 7 Democratic seats in the toss-up column (more so than any of the three prior elections at this point). 13 Democratic seats overall are in the competitive columns, while only 5 Republican seats are. If Republicans do any better than they are currently projected to in either polling or expert opinion, then it could be a long Election Night for Democrats.
That said, the general lack of correlation between early expert analysis and final results and 2008 polling errors should read "caution Will Roger". Nothing in the Senate is set in stone. Still, I would put early money on a good Republican night.