What is the argument in The Semisovereign People by Schattsschneider?

In this book, written in 1960, Schattsschneider looks at the assumptions people have regarding the way a democracy works. He writes, "The unstated premise...is that the people really do decide what the government does on something like a day-to-day basis" (page 130). However, this definition of democracy ignores the role of the political parties and pressure groups in defining the scope and bias of the policies that are fought over in the public arena. 

The author defines the difference between party politics and pressure politics. In examining the differences, he states that party groups are large-scale organizations, while pressure groups are small-scale organizations. In looking at the scope and effect of pressure groups, he further suggests differentiating between private interests and public interests. He defines special interest groups as organized. He states that "the business or upper-class bias of the pressure system shows up everywhere" (page 31). For example, business people are four to five times more likely to write their representative in congress than are manual laborers (page 31). He concludes, "the pressure system has an upper-class bias" (page 32). People of lower socio-economic status (SES) are much less likely to take part in voluntary organizations that play a role in the pressure system.

As he writes, these data suggest that special interest groups are not a "universal form of political organization reflecting all interests" (page 34). Not everyone can take part in these groups because they're small by definition. Here is the crux of his argument: "The flaw in the pluralist heaven is that the heavenly chorus sings with a strong upper-crust accent" (page 35). This is the problem with a pluralist system--people of high SES are well represented in pressure groups, while other people are not. The idea that pressure politics reflects the interests of the entire community is false. However, public authorities play a role in moderating the power of the rich over the other members of society and do not merely allow more powerful pressure groups to win.