(FORTUNE Magazine) – Diet Pepsi's new advertising campaign featuring celebrity achievers has reaped a sweet harvest of free publicity for Pepsi-Cola USA. The spot featuring former Democratic vice presidential candidate Geraldine Ferraro and her daughters--for a fee reported at anywhere from $75,000 to $750,000--easily upstaged rock crooner Lionel Richie's new Pepsi ads. Most of the media coverage centered on the propriety of politicians as product promoters. Politicians have rarely appeared in commercials and then only if they no longer held office. William Miller, the 1964 Republican vice presidential candidate, and Sam Ervin, former North Carolina Senator and star of the Watergate hearings, appeared in American Express's ''Do You Know Me?'' commercials. Recently former Republican Senator Howard Baker, a possible presidential candidate in 1988, did a commercial for USA Today. The potential conflict of interest makes most politicians skittish about pitching a product, and advertisers haven't been exactly beating a path to their door anyway. ''In 99% of all cases, I would not use a politician to sell a product,'' says Tony Schwartz, who produces commercial and political ads. ''I don't think they have credibility in that context.'' Pepsi executives say they chose Ferraro because of her achievements rather than her political views and that they might use other politicians. ''We would look for a person who represents something more than a personality,'' says Roger Enrico, C.E.O. of Pepsi-Cola USA, ''someone who has done something that had never been done before.''