The TalkBackers here have been crying out for insight on which Democratic candidate helps vulnerable down-ticket candidates more. As an experiment, I looked through who the Bush 63, or the Democratic Congressmen who occupy districts Bush won in 2004, have endorsed*. (Districts that went for Bush in '04 is an imperfect measure of vulnerability, but it's the one the NRCC uses to highlight who's vulnerable.)
The result? Most of these people haven't said mum, probably because their coalitions are so fragile they can't afford to piss off any HRC or BO supporters off. But Hillary has slightly more endorsements within this vulnerable group, 14 to 12, which undercuts the conventional wisdom that down-ticket candidates in red regions don't want her as the nominee.
That said, if you take out the Congressmen from Arkansas, New York, and Illinois, Obama gains the edge in red-district endorsers, 10 to 7. Furthermore, if you take out Hispanic Congressmen, Hillary has only two vulnerable endorsers left (Leonard Boswell in Iowa and Darlene Hooley in Oregon. I can't take black Congressmen out of the equation because there are none that represent Bush districts!).
The upshot? Hillary has marginally more support from people whose districts went red last presidential election, but it's more limited to specific groups: Arkansans, New Yorkers, Hispanics. Obama doesn't have anywhere near overwhelming support among red-district Members, but the support he does have is somewhat broader than Hillary's, comprising Congressmen from South Dakota, Washington, West Virginia, Texas, Minnesota, etc.
*This Hill list of congressional endorsements is nearly up-to-date, but not quite.
Update: Here's my data.