By definition, negative campaigning can't happen in a debate. The other guy is there to answer. There has been surprisingly little negative advertising in either party's nomination fights so far. Negative campaigning obscures the source of the attack whenever possible. The best places to do this are radio and TV ads and direct mailings.
Negative ads are sneaky.
Federal laws require a candidate who buys any political ad to identify himself, but this legal requirement will be met as anonymously as possible. Nowhere in the body of the ad will the allegations against the attackee be attributed to the sponsor of the ad. Smith's moral inelegance will always stated as a fact. That law is a joke. Keep it simple.