It’s pretty obvious why special elections, called when a member of the House resigns or dies, are poor indicators of anything. First of all, they’re hardly a representative sample of anything; House districts are rarely closely split between the parties, so most of what happens in specials is the incumbent party winning thanks to a partisan edge. On the other hand, since incumbency matters in House elections, it’s not unusual to find a seat held by a member despite a partisan disadvantage, and those seats will naturally tend to flip the other way once the incumbent is out of the picture. None of that, of course, tells us anything about national trends.

Moreover, special elections almost always have very low turnouts, and they often have odd rules — candidates may be chosen by a small group of formal party officials instead of in primary elections, or there may be a free-for-all format in which one party may field one candidate while the other has three, as may be about to happen in Nevada’s second district. Add it all up, and Aaron Blake is exactly right: These elections are overhyped and shouldn’t be taken too seriously.

And yet . . .

One thing we do know about congressional elections is that candidate quality matters, and that the strongest candidates are usually experienced politicians who make choices based, among other things, on their estimates of whether it’s a good year to run. And how do they arrive at those estimates? Well, in large part, most likely, the same way that the rest of us do — by assessing obvious indicators (such as presidential approval ratings) and by tapping in to the conventional wisdom. Which, in turn, is going to be shaped by whatever it is that pundits and professionals are talking about. Which, this morning, was at least in part the upcoming NY-26 special, and will soon be the NV-2 special, with the likely participation of news magnet Sharron Angle.

Right now, dozens of state legislators, small city mayors and other potentially strong candidates are trying to decide whether to run for the House in 2010.  If they collectively decide it’s going to be a good year for the Democrats — or a good year for the Republicans — that will help determine what they’ll do. And if all the strong Democrats run while all the strong Republicans wait for a better chance, then the odds are that it will actually turn out to be a good year for the Dems, thanks to all those strong candidates. It’s also true for other actors, too: potential donors may choose to give now or wait for another year, and activists may decide to get involved or believe that it’s pointless.

Partisan congressional tides, in other words, are among other things partially a result of self-fulfilling prophesy, and special elections can certainly be a significant piece of what establishes the prophesy in the first place. It’s an odd kind of importance, but it does make them more important than they might seem.