Negative ads are used to convince soft supporters of the other side that their candidate/party is not worth the effort to make trip to the polls. No minds have been changed, but that non-trip is almost as good as a vote for our side.

If the ads (positive or negative) didn't work, the ad-men would not be using these political ads. Let's just assume the ad men know what they are doing to get votes or scare votes--most of the time. And they need only shift voting intentions a little to attain a successful result.