David Brooks speaks of "negativity and cheap-shot campaigning". But it's worth noting that these are two very different things. Cheap shots obviously detract from the quality of our politics, as does gratuitous negativity and demonization. But, just as obviously, not all negativity is unwarranted. Sometimes -- no, often -- other politicians are up to no good, and it's important and worthwhile to draw attention to this. (How else are we to hold them to account?) Negativity is entirely appropriate in response to substantive flaws or wrong-doing.
How is it that people so often fail to appreciate such a perfectly obvious point? Perhaps it is a side-effect of popular subjectivism. Since all perspectives are "equally valid", there's no distinction to be made between legitimate and illegitimate criticism. There's "no truth of the matter," so whenever people disagree they must simply be trying to impose their own will by means of verbal force. It's a game that involves only emotions, not reasons. 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.