Here is an interesting analysis of the best- and worst-case scenarios for Democrats in 2010 by FiveThirthyEight.com blogger Nate Silver. As he explains, these are not necessarily the most likely scenarios, but each one is very much on the table.
The best scenario first:
A couple of weeks ago, we examined the potential upside case for Democrats in November’s midterms. If the party were able to limit their losses to about 20 House seats and 3-4 Senate seats, it might not have as deleterious an effect on their policy agenda as you might think.
But that is the upside case for Democrats. It is not the base case, and it is certainly not the worst case — both of which look as grim as ever. Although I think people may somewhat underestimate the probability of a shift in momentum back toward the Democrats, they may simultaneously be underestimating the magnitude of losses that might occur if momentum fails to change, or moves in the other direction.
The worse case scenarios now:
In my piece a couple of weeks ago, I wrote that there was only a 1 in 10 chance that Democrats would lose more than 55 seats in November. Having now looked at this issue in somewhat more detail, that clearly seems to be a lowball estimate. While there is other statistical and anecdotal evidence that one can point toward that is relatively more favorable to Democrats, and while there are other techniques, like a district-by-district analysis, that could be applied to this problem instead — if you could get 9:1 odds (a 1-in-10 chance) on the Democrats losing more than 55 seats in the House, that would be a good bet.
And what if, for example, the Rasmussen case comes into being. Rasmussen has the Democrats losing the generic ballot by 9 points (and has had similar numbers for awhile). A 9-point loss in the House popular vote would translate into a projected 65-seat loss for Democrats. Or, if we adjust the Rasmussen poll to account for the fact that the Democrats’ performance in the popular vote tends to lag the generic ballot, it works out to a 12.4-point loss in the popular vote, which implies a loss of 79 seats!”