If you're trying to convince an audience, it matters how you treat the person you're arguing with. Don't be a jerk. People notice that, and judge you for it. They judge your position for it - perhaps not intellectually, but emotionally.

The best possible thing to do in that situation is to out-evidence them, out-argument them, and out-nice them. And really, if the facts are on your side, you shouldn't have to be a jerk or manipulative.