This story suggests an important dynamic that political scientists have been aware of for some time. The larger question is, what explains substantial shifts in the partisan balance in Congress? The obvious answer is that the preferences of voters change, and that, regardless of who is on the ballot, they vote for a different party than they did last time. Some of this is obviously part of the story, but this “demand side” explanation can’t be all of it.
An alternative explanation looks instead at how expected shifts in voter preferences create changes on the supply side. Think of it this way. Twelve months out, potential candidates for Congress start sizing up what they anticipate the conditions will be a year ahead. If conditions for their party look good, incumbents will choose to run again, and quality challengers decide that “this is the year” and enter the race. On the other side, if conditions for your party seem to be bad, some marginal incumbents decide that “this is the year to hang up their cleats,” while quality challengers decide to wait a few years. Given the uncertainty about future voter preferences, some incumbents and challengers may also use the behavior of others as cues for their own decisions, which can give the process a self-reinforcing quality.
What is interesting about this process is that even if voter preferences return in the next twelve months to where they were in the last election, expected voter preferences today could have a major impact on the next election, by altering the distribution of quality candidates. This has got to make Republicans worried, since it suggests that if things don’t change very soon, they will have dug themselves a hole on the candidate quality front that it will be impossible to extract themselves from, no matter how much progress they can make on the issues in the next twelve months.
UPDATE: Mark usefully suggests an additional consideration. The value of being a member of Congress is strongly connected to whether you will be in the majority or minority, which magnifies the dynamics above. If you’re currently in the majority, but believe that conditions a year from now will be bad enough that, even if you are re-elected, you will be in the minority, it gives you a strong incentive to retire. The same is true, but inversely, for a potential challenger–if you have the potential for being in the majority, it increases your inclination to run. In fact (and in honor of Schelling), these dynamics have a strong “tipping point” phenomenon–propensity to run will shift proportionally to changes in perceived party strength, up to the point where members of Congress (and potential challengers) believe that there is a reasonable probability that party control will shift. At that point, even small declines in perceived party strength may lead to large numbers of majority members retiring which, because it makes it even more likely that the party will lose control, will lead to additional retirements (as well as inducing new high-quality challengers to make the race).
There’s a cite for this, but you’ll all have to wait until I remember it!