The owner, licensee or operator of a radio and television broadcasting station or network of stations, and the agents or employees of any such owner, licensee or operator, shall not be liable for any damages for any defamatory statement published or uttered in or as a part of any such broadcast, by one other than such owner, licensee or operator, or agent or employee thereof, unless it shall be alleged and proved by the complaining party, that such owner, licensee, operator, such agent or employee, failed to exercise due care to prevent the publication or utterance of such statement in such broadcast; provided, however, that in no event shall any owner, licensee or operator, or the agents or employees of any such owner, licensee or operator of such a station or network of stations be held liable for damages for any defamatory statement broadcast over the facilities of such station or network by or on behalf of any candidate for public office.