A reader emails an addendum to my worries:
The main way the interests of partisan incumbents on both sides converge is in drawing Congressional district lines. There are very few competitive districts, which means in any given district (a) the incumbent has a tremendous advantage (incumbents in the House are re-elected well over 90% of the time), and (b) if there's no incumbent, the advantage lies with whichever candidate has the strongest appeal to the majority party's base -- not to swing voters or moderates of both parties.
Think about the math behind this, and you'll see why 2006 was the exception and not the rule.
Most 2006 districts were drawn by Republicans, as they controlled a majority of state legislatures pre-'06. Their aim was to maximize the number of majority-Republican districts, but obviously this required slicing their majority as thinly as possible in each district. Public sentiment swung far enough from the norm in '06 that it exceeded the margin of safety the line-drawers had thought sufficient, and since this margin was similar in many districts, the result was a huge electoral swing for the Dems. Same principle as the Titanic's watertight bulkheads going only partway up the hull -- once one was overtopped, they all were.
I was disappointed that Schwarzenegger's proposal to have district lines drawn by a panel of retired judges, instead of the legislature, was rejected by California voters. I think they were sufficiently outraged by his other proposals on the same ballot, and sufficiently bored by the arcanity of this issue, that they just threw the baby out with the bathwater.
So long as legislative district lines are drawn by partisans, legislatures will be dominated by extremists.
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