I've just read a book called "Fooled by Randomness" by Nassim N. Tabeb (interesting subject matter but the author irritated me no end....)
Anyway, one of his good discussions covers a field called heuristics and the phenomenon of how our emotional brains and our logical brains interact. We need both, but there are different messages that appeal to one kind of thinking rather than the other.
What I see the Republicans doing is direct appeals to the emotional brain. They have an extremely weak case to make to our rational brain side, so they ignore it.
Obama has to be very astute in providing his own emotional brain appeals. His messages need to both attract people to him and counter the Republican's negativity without giving them any new material to work with. His rational brain messages are all just fine.
(I'm going to read Taleb's new book too - "The Black Swan" - so I guess he didn't damage my emotional brain too much....)