Anyway, there’s a pattern here: Coakley carries districts where Obama got 65% or more of the vote and runs essentially even in the district where he got 64%, and Scott Brown runs ahead in districts where Obama got less than 64% of the vote.

Let’s extrapolate those numbers to the nation as a whole and assume that a district that voted 64% or more for Obama is safe for Democrats even under the most dire of circumstances. How many such districts are there? Answer, according to this source: 103. The other 332 districts voted 63% or less for Obama. Interestingly, there are more 64%+ Obama districts in the West (36) than in the East (27) and more in the South (21) than in the Midwest (19)…

So that means that 101 of the 256 House Democrats represent 64%+ Obama districts and that 155 House Democrats represent districts which might, according to the Massachusetts metric, be vulnerable in some circumstances to Republican capture. No wonder so many House Democrats refused to vote for the Senate health care bill—enough to prompt Speaker Nancy Pelosi to say publicly that “unease would be a gentle word” to describe their attitude toward doing that.