the reason to be worried is simple: Obama is losing popularity in monumental chunks. If he goes at this rate he will get to Bush's popularity ratings level before the end of the year. And this is without katrina, iraq, corruption or anything else. People are really starting to hate democrats and democratic brand. Now clearly, the "ringleaders" of the operation are right wing groups. They channel people's dissatisfaction with what is going in the country/world toward democratic brand.
Even if Obama is reelected but loses democratic congress, it will be a lot less fun for him. A thing called impeachment (for any BS reason whatsoever) becomes a possibility. All kinds of investigations could get launched and basically it is a nightmare.
Democrats need to go with moderation and stay away from steering hard left.