Political Attack Ads
If you say "politician A is a crook" often enough, it is only a matter of time before the public comes to believe that all politicians are crooks. That is what is happening now and these are the seeds that defenders of negative advertising are sowing. Politics is not about bludgeoning your opponent until they cannot stand.
Whether or not you view a political ad as "negative" often depends on your partisan political point of view. The NDP recently released a negative T.V. attack ad that is so blatantly negative it almost comes across as an attack ad parody. Yet when I went on Twitter and described it as such, I was immediately met with protests. How can you ban something like attack ads when political partisans don't believe they exist?