I don't know if you've ever noticed people fighting, but when they are both screaming at each other you tend to dismiss them both regardless of the merits. When one appears to be acting reasonable you tend to side with them just a touch (even if on the merits they aren't as grounded).
I'm sure this is true (as a general tendency), but it seems kind of unfortunate. I've noted before that it's easier to seem reasonable than to actually be reasonable, i.e. to aptly judge issues on their merits. Further, if we tend to think badly of others for being negative, no matter how warranted their criticisms might be, that creates an unfortunate incentive against critical thought. Negativity is entirely appropriate in response to substantive flaws:
[According to popular subjectivism:] Criticism is mean and nasty, something only bad people engage in. Nice people are always happy and co-operative, appealing to our positive emotions rather than negative ones. So the story goes.
Once we reject subjectivism, however, a better alternative presents itself: not 'be positive', but be reasonable -- do what the situation calls for. If there is good reason to criticise the opposition, then do so. Otherwise, don't. Simple.
The upshot: you can't just complain that the other team is engaging in 'negative campaigning'. There's nothing wrong with negativity per se. The real question is whether their negativity is justified: i.e. whether their claims are important and true.
On the other hand, hysterical screaming does tend to correlate with substantive lunacy, so perhaps it's not wholly unreasonable to be prejudiced against angry-sounding screeds (at least until you're in a position to assess the actual content).
But it would seem rather unjust to think badly of someone who was actually clearly in the right (i.e. clear to anyone who bothered to examine the dispute on its merits). So should one instead suspend judgment if they can't be bothered assessing the matter more carefully? Should our attitudes towards others be governed by the principle of 'innocent until proven guilty'? Better a thousand fools be free of your inner censure, than one good person wrongly maligned? Or should we maximize the likelihood of true belief by engaging in statistical discrimination (generalizations) and using whatever evidence we've got?
Or maybe it doesn't really matter how we judge others? They're just thoughts.
Even so, better to have one's thoughts be accurate, no? (Perhaps it only matters when one will act on said beliefs, e.g. by supporting or condemning a disputant. In that case, it seems much clearer that one should take care to judge the matter aright.)