The House: Too Few Hot Races or Tidal Wave? Hard Numbers
by jay l, Thu Jan 12, 2006 at 06:26:06 AM EST
There are two views floating around on whether we can take the House. The first view, offered most notably by Charlie Cook (see http://www.cookpolitical.com/...) is that there just aren't enough competitive races; realistically, the Dems can pick up 4 to 9 seats. The counterargument (I believe Matt has posted on this) says this misses the forest for the trees. If this is a "tidal wave" election analogous to 1994 (similar dynamics if not the same magnitude), races that don't look competitive now will be, and races that look competitive could go blue en masse.
Well, I've crunched a lot of numbers. And, I think I've got a good handle on what kind of aggregate margin we need to take the House. (For those who can't wait, its around 5%.) I started by studying aggregate vote margins over the past sixty years and how they correlate to the allocation of seats. (The raw data is available at http://clerk.house.gov/....) For example, in 2004, 113 million or so folks voted in House races. Rs took 49.2% of the aggregate; Ds took 46.6%. So, the Rs aggregate vote margin was 2.6%. The Rs have 232 seats to 203 for the Ds, for a 29 seat margin. So a 2.6% vote margin translates into a 29 seat margin. When you do this for the past thirty cycles, here's what you find out:
1. Only once has a party won the aggregate vote and not controlled the House (1996, when Ds had a 0.3% vote margin but Rs controlled the House by 17 seats.)
2. On average the ratio of the seat margin to the vote margin is 11.1. I.e., a one percent margin in the aggregate voting translates into about 11 seats.
3. If you use the 11.1 ratio, to predict seat margin from vote margin, you can then look at how close the actual seat margin is to the predicted seat margin. For example, in 2004, we would have predicted a 29 vote seat margin (2.6 x 11.1 = 28.9). The variance in this case was 3.
4. The average variance since 1946 has been 19. So, based on this and nothing more, you could say that if the Ds take the aggregate vote by 2%, there is a 50% chance that they will have a seat margin between 3 and 41.
But, there's another way to look at the data. We can look at the change in voting margin from one cycle to the next and ask how that translates into seat pickups. For example, if the Ds take the aggregate vote by 2%, that would be a 4.6% increase in voting margin from 2004 to 2006. It turns out that on average a 1% increase in voting margin translates into a pickup of 3.2 seats. By this metric, the Ds would need a 4.7% improvement in voting margin to get an expected pick up of 15 seats. In other words, we need a 2.1% margin in the aggregate vote to have a 50% shot at taking the House.
If that was where the story ended, I'd be a pretty happy camper. But, in fact, the news is not quite so good. I also looked at data on the voting margins in each House race in 2004. These numbers are much less encouraging. One way to cut the numbers is to rank order every district by the voting margin in 04. Start with the Ds who got 100% and keep going until you get down to Rs who got 100%. Since the Rs have an aggregate vote margin of 2.6%, you might think that the median vote margin - number 218 on the list - would be an R with a margin of 2% or 3%. No. It's an R with a margin of 10.5%. The reason is that the Ds "waste" a lot of their votes in districts that are extremely partisan. Some of that happens with Rs as well, just less so. For example, 99 Ds have a vote margin over 40%. Only 70 Rs. This gives the Rs a significant structural advantage that is not reflected in the analysis above. As a result, to pick up the house, Ds will have to win a lot of races that Rs won in 2004 by 8%, 9%, 10% or more. Using the R+10.5% figure (median individual district vote margin in 2004), suggests that perhaps to take the House we need to increase our aggregate vote margin by 10.5%; i.e., that we would need an aggregate vote margin of 8%. I don't think this can be right. For one thing, an 8% vote margin would predict an 88 seat margin. If in fact, it resulted in only a one seat margin, that would be a variance of 87. The largest variance in the last sixty years was 50. Also, I think that if we increased our aggregate vote margin by 10.5%, the increases would disproportionately show up in the more competitive districts (as opposed to places where candidates run unopposed). In other words, I suspect that a 7% increase in aggregate vote margin would show up as 8% to 10% to 12% increases in the more competitive seats.
So, we've come up with two figures on what aggregate vote margin we need to take the House. The first was around 2%. The second was around 8%. I think the truth is in the middle. I think we need around a 5% aggregate vote margin.
Ok, suppose that's right. How realistic is that? Pretty realistic I think. A poll of polls on the generic congressional polls shows Ds up by 10% or so. I don't think we'll sustain that. One thing I'd like to study more is how early GCPs have compared to final GCPs. My gut is that we're likely to see that number go down fairly significantly over the summer and fall, just based on the Rs engaging in battle, spending money on advertising, stoking their base, etc. But, the fact that we're at +10% or so now suggests that 5% or even a bit higher is very realistic. And, by the way, see this Pew study http://people-press.org/... on the impressive accuracy of GCPs in off-year elections.
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