Not all of the 31 Democrats who signed the letter to Pelosi yesterday are on the chopping block, but a large majority of them are from rural, red districts. In fact, the rural advocacy blog Daily Yonder found that two-thirds of the nation's closest races in 2010 are in rural areas.
Intuitively, this makes sense. Rural areas are more conservative, and Democrats made inroads in those areas in 2006 and '08. From the Eastern Shore of Maryland to the High Plains of Colorado, those incumbents are now up for rehire. But even in historically Democratic rural areas like the Bayou of Louisiana or the Upper Peninsula of Michigan, Dems are in trouble.
Some of Democrats' legislation, such as cap-and-trade, has certainly been unpopular in places like Danville, Virginia and Waco, Texas, but I don't think that tells the full story.
I think consciously or unconsciously, voters see this as the most urban government in history. With a president from Chicago, a speaker from San Francisco, and two new Supreme Court justices from New York City, there may just be a simple lifestyle disconnect between government and voters.