Michael Barone  has taken a look at the results of the Massachusetts Senate race on a district by district basis (among the Mass congressional districts) and notes the following:
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).
All but two of the 103 Obama 64%+ districts are represented by Democrats. The two exceptions are Louisiana 2, where Republican An Joseph Cao beat Democrat William “Cold Cash” Jefferson in a December 2008 runoff, and Florida 19, whose incumbent Robert Wexler resigned and a special election will be held in April. And, yes, it will be amazing if this heavily Jewish district in Palm Beach and Broward Counties elects a Republican; heavily Jewish Brookline and Newton voted heavily against Scott Brown in Massachusetts.
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. ...
Indeed. Of course this is just numbers, and in now way saying that this is the kind of result we would be looking at in the mid-term election, but the bottom line politically is that you've know got a LOT of Democrats having night sweats about this coming November.