The Democrats are doing a great job selling themselves as the party of hope, growth, and opportunity--the optimistic people who believe in American greatness and the American future. If they're lucky, George W. Bush will replay Bob "Bridge to the Past" Dole's 1996 message. I doubt that he's that dumb, but his opponents have certainly put him in a box. It's hard to go negative, or even talk about tough issues like war and terrorism, without sounding scary, mean, or gloomy.
Unlike the challengers, who can speak in generalities, the incumbent needs to lay out specific vision that gives the public a picture of the future, and his future policies, that goes beyond "more of the same." That's especially true at the moment. If people want vague promises that things will be better, they'll go with Kerry.
One plus, or at least the absence of a minus, for Bush is that the Republican base has mostly recovered from the insane cultural pessimism that seized it in the mid-90s (pricelessly portrayed in Rich Karlgaard's Reason report from Dark Ages Weekend). Now it's the hard-core Democrats who think the country is going to hell--but at least they blame the administration, not the general public.