"The subtlest and most pervasive of all influences ere those which create and maintain the repertory of stereotypes. We are told about the world before we see it. We imagine most things before we experience them. And those preconceptions, unless education has made us acutely aware, govern deeply the whole process of perception." - Walter Lippmann, "Public Opinion", page 89.
So who creates and controls these preconceptions? Journalists do, and he knows it. He says so on page 355:
It is a problem of provoking feeling in the reader, of inducing him to feel a sense of personal identification with the stories he is reading. News which does not offer this opportunity to introduce oneself into the struggle which it depicts cannot appeal to a wide audience. The audience must participate in the news, much as it participates in the drama, by personal identification. Just as everyone holds his breath when the heroine is in danger, as he helps Babe Ruth swing his bat, so in subtler form the reader enters into the news. In order that he shall enter he must find a familiar foothold in the story, and this is supplied to him by the use of stereotypes. They tell him that if an association of plumbers is called a "combine" it is appropriate to develop his hostility; if it is called a "group of leading business men" the cue is for a favorable reaction.
It is in a combination of these elements that the power to create opinion resides. Editorials reinforce.
This is how the game works. The journalist knows that most people are too busy in their daily lives to take their time to learn about the association of plumbers. This lack of education is key to their power to manipulate, so the journalist can insert faulty education in its place and then use it against the reader. That's exactly what he says. The journalist educates the reader that the plumbers are a "combine", if the journalist wants the reader to hate the plumbers. That is a "subtle and highly pervasive influence which has created and will now maintain this stereotype".
The reader's preconception of the bad plumbers governs deeply the whole process of perception.