The separation of candidates and Super PACs allows the candidates to simultaneously disavow and benefit from Super PACs generated attack ads. “What’s developed is this division of labor where the candidate’s campaign has the ability to focus more of its advertising on positive messages because they can leave the negative to the Super PACs,” said Anthony Corrado (government).

See Also:
Super-PACs Dominate Airwaves with Flood of Negative Commercials
Bloomberg
http://www.bloomberg.com/news/2012-01-18/super-pacs-dominate-airwaves-with-flood-of-negative-commercials.html