Three political scientists have produced a paper on the relationship between the generic ballot and actual seats that is definitely worth a read. Here are the choice bits:Now consider the polls over the final thirty days of the 2006 campaign. As of early October 24, PollingReport.com listed the results of 6 likely-voter generic ballot polls conducted during the final 30 days of the campaign, by CNN (2), USA Today/Gallup, ABC/Washington Post, Fox/Opinion Dynamics, and Newsweek. The average Democratic two-party share in these polls is 57.7%. Applying our formula as described above, the Democrats should win 55% of the two-party vote with a confidence interval from 51.3 to 58.7 percent, implying that the Democrats almost certainly would win a majority of the votes cast.

But would this mean that the Democrats would also win the most seats? If the Democrats were to win 55% of the vote, this would represent a 6.4 percentage point swing from 2004, when they received 48.6%. If Democrats were to win exactly 6.4% more of the 2006 vote in every district than they won in 2004, they would win 228 seats. However, an average swing of 6.4% percentage points will be spread unevenly--sometimes more than 6.4% and sometimes less. We must take the degree of uniformity into account. Moreover, as we already discussed, the prediction that the average vote swing will be 6.4% is itself subject to error. We take these considerations into account by a set of simulations described below. The simulations suggest that a predicted national vote surge of 6.4 percentage points would yield the Democrats 235 seats, for a 32-seat gain. This is 7 seats more than with uniform swing.(...)

Thus we see that the Democrats need more than the plurality of the generic vote in order to expect to control the House. In the range where the Democrats have between 50 and 52 percent of the "two-party" generic vote, majority control is up for grabs. But if the Democrats have 53% or more of the vote by the generic ballot question, as they do as of this writing, they are the heavy favorites (probability > .90) to win the House of Representatives.(...)

If current trends in the congressional generic ballot polling persist (which they have in past election campaigns), the Democrats are near certain to win control of the House. But this assumes a continued Democratic lead of 8 or more points among likely voters in the generic ballot. If the lead dips below this level, the Republicans can rekindle their hope of holding the House. So, an eight-point lead or more is what we are looking for in the generic congressional ballot. Interestingly, I have always had that same number, an eight-point Democratic lead in the generic ballot, in the back of my head: I have a number in my head, 43.0%, that serves as the dividing line between a potential wave election for Democrats and an election where our best hope is for narrow control. (Don't ask me how I came to that number, I just have.) In the generic ballot, I'll stay reasonably confident as long as our five-poll average lead stays over 8.0% My subconscious mind might be smarter than my conscious mind, if it can make a calculation like that.

The system of damns in levees that Republicans have in places (maps, voter suppression, fundraising, media, GOTV) appears incapable of withstanding a blow of grater than eight percent. In what had been a fairly static, evenly divided nation from 1992-2004, that advantage of up to eight points was overwhelming, as we slowly depressingly learned over the past decade. That advantage was also why narrowly targeting the swing via triangulation was a nearly hopeless crusade for Democrats, since we needed a big win to break the system. However, with a wave of greater than eight points, not only can we take control, but we can also smash much of the Republican power system. We can dismantle the K-Street project, redraw maps, shift the media, shift fundraising, make real electoral reform--the works. We can significantly reduce the eight-point, built in Republican advantage by taking control, which will come in very handy should the nation revert to an even split. So, when you are watching generic ballot polls, keep the number 8 in mind. Anything above, and we should have blue skies up ahead.