Yahoo's online advertising partnership with newspapers is facing a new threat — from the newspapers themselves. Five of the nation's largest newspaper companies — Gannett, Tribune, Hearst, MediaNews, and Cox Newspapers — are teaming to create a one-stop shop for online advertising. A single sales force will be able to sell ads across all major markets. Hearst, MediaNews, and Cox remain members of the Yahoo consortium, but the new partnership is foreboding, especially for Yahoo president Sue Decker, who helped engineer the deal and keeps holding it up as a totem of Yahoo's new partnership strategy.