The problem of course is not necessarily the attack but HOW the attacks are done. One can attack and still have a positive campaign.
Point out issues and differences in approach and give your own view on it. Compared how policies doen by one has failed and comparitively how the policy done by another has suceeded. Talk in more generalities when speaking about negatives, and keep it issue focused rather than veering off into either more personal or barely relevant things (like the birther issue) or into huge hyperbole (Will destroy our country, hates grandma, etc etc). And overall be positive in your message, positive of what America can be, of what America is, of the people. Make it out that your platform and agenda is going to fix things and make things better RATHER than focusing primarily that the opponents platform and agenda have made things worse. People know and realize things are bad, you don't have to retell them that. They will respond much better to you telling them it can get better rather than reminding them directly that its bad.
If you're an aquantince and I'm offering you a job, and I know you're being paid jack crap in your current job and its not helping you get your debt paid off I could go one of two ways (or a mix of both I guess). I could remind you that your current job is paying you crap and you're going more in debt and you should come work with me instead. OR I could tell you that you'll be making more with me and before long the extra money will help you get out of debt.
In one case, peoples natural reactions to become defensive of what is the norm comes into play. The natural reaction to immedietely see you as a critical and negative person also comes up. You also immediete put the individual into a negative frame of mind reminding them of their problems.
In the other case, a persons natural reaction to better themselves is brought more into play. You're more likely to be seen as a generous and likable person. You immedietely put that individual into a more positive frame of mind thinking about how much better things could be.
In both instances, you're attacking their old job. It just happens in one case you're doing it in a directly negative way by specifically mentioning that the job pays like **** and is leading to them being in debt. In the other case you're doing it in an indirect way by playing on the fact that the individual KNOWS they're being paid crappy already and implying you're better than that other job because you'll pay them better.
One can run a POSITIVE campaign while still going on the offensive. A positive campaign does not have to mean an attackless campaign. Indeed, its not just unwinnable...its almost impossible to run a political campaign against someone without identifying how you're different and why you're better, which is indirectly implying that the other person is bad (or at worst, worse than you).
