What’s the Amplitude, Karl?

So Karl Rove has whipped out “the math” once again to point out that the average number of seats gained by the “out party” in the mid-term year of a president’s first term in office is 23. Eh. First off, in the ten elections which fit this description, the out party has only gained 20 or more seats four times. The median, as you might guess, is a good bit lower – just 17.

But I think reaching back in time as far as Rove does is misleading. Now, Rove of course is always misleading, but this time he’s conning members of his own party into thinking 23 is their due. I’m happy to let them keep believing him. But you, dear SSPers, are smarter than that. And this graphic is all you need:

The above chart shows how many seats the Dems gained or lost each year, going back to 1918, the year in which the direct election of senators began. As you can see, the further to the right you go – ie, the more recent you get – the smaller the amplitude of the curve. In plainer English, the size of the swings from cycle-to-cycle has gotten smaller over time.

Put another way: In the 34 elections from 1918 through 1984, only six times did we see single-digit changes in the number of Democrats in the House. In just 12 elections from 1986 to 2008, there were single-digit shifts nine times. During that time period, the average shift has been less than 13 and the median more like 7.

Still a further way of looking at the phemonenon in the first chart is this:

There are a lot of ways you can slice this apple, but I’ve chosen the simplest. This is simply a running average of the swing in House seats going back to 1918. (That is to say, each point on the line represents the average from 1918 to that date.) The trend is unmistakable – the average swing has been inexorably shrinking for a very long time.

The reasons for this change are multiplex and endlessly debatable. They include, among other things, a mixture of more sophisticated gerrymandering, intensive incumbent-protection programs, and the sharpening of ideological boundaries between the parties following the post-Voting Rights Act realignment.

With this realigning shakeout largely concluded, it’s difficult to see many more wave elections in the near future. If you want to let ancient history be your guide, as Karl Rove does, then sure, big swings look eminently possible, even likely. But the recent past is much more informative, and it looks like we’ve settled into something resembling a pattern of taut stability.

We may yet lose seats in the House in 2010, but I very much doubt as many as the Rover thinks we might. And this almost certainly means that the Republican Party has a difficult slog ahead of it for a long time to come.

UPDATE: I’ve created another chart, this time of the ten-cycle moving average starting in 1936. It’s a little bit “noisier” (not surprisingly), but it also shows the same clear downward trend in swing size: