barack obama's raising money for robin carnahan. why? >> i need another vote. it'd be helpful. >> he knows robin carnahan will rubber stamp the pelosi-reid- obama liberal agenda: >> woodruff: which highlights the question some vulnerable democratic candidates are surely asking themselves-- can a push from the president do more harm than good? former republican congressman tom davis says it depends on where they are. >> the president maintains a strong popularity among african americans. to get a democratic base model that helps elect democrats, he can be very helpful. on the other hand, among independent voters, particularly in the south and the midwest, and the west, at this point, he's probably a liability. and candidates want to run showing their independence, not the fact that they will be obama robots. >> woodruff: richard norton smith agrees it's a tricky question for democrats facing serious challenges. >> its a question of the level of visibility. how visibly do you want to be associated with the president? you certainly want to be associated with the p