AP may have provided the model, but the partisan papers continued long after AP was organized in 1848. It had more to do with the rise of journalism schools, those great cleansers of the public prints, in the early 20th century, coinciding with the first great wave of consolidation of newspapers and establishment of chains as they became more profitable, by people such as Frank Munsey, Joseph Pulitzer and William Randolph Hearst.
Remember that Hearst newspapers built up their circulation as pro-labor sheets until the New Deal actually started allowing unions to organize. Then Hearst switched his papers to a more conservative viewpoint.
A.J. Liebling went over this ground brilliantly in "New Yorker" writings collected as "The Wayward Pressman."
By the way, AP membership used to be exclusive to one newspaper in each town. That's why Scripps-Howard started United Press and Hearst started International News Service.