Paul Krugman calls them "Clinton rules," where everything a candidate says is distorted by the other side into a negative ad. Negative campaigning in general sickens Krugman and warns the DNC "...if history is any guide, if Mr. Obama wins the nomination, he will quickly find himself being subjected to Clinton rules. Democrats always do." (Hate Springs Eternal, February 11, 2008)
It's not clear which party engages in more negative campaigning, but I bet it has a lot more to do with the desperation of the campaign than its party. Despite what the electorate claims, negative campaigning works and is especially tempting in the political sphere where the runner-up is still a failure. There is so much to lose. Recall one of the most famous (and most negative) campaign ads in U.S. history, where Democrat Lyndon Johnson suggested Republican Barry Goldwater would be responsible for a nuclear war if he was elected. Mr. Krugman should read his own titles: hate truly does spring eternal.