paper on exclusivity in American politics

I have a class assignment to do a critical analysis of "By Invitation Only," 
a book by Stephen Schier, in which he argues that with the proliferation of 
interest groups and the role of communication technologies, participation in 
American politics has become more exclusive, owing to the decline in partisan 
politics over the past 100 years and especially with the rise of interest 
groups in the 60s.  Nowadays it is more rational for campaigning politicians 
to activate a small segment of the voting population than to use strong party 
messages that appeal to larger groups.  Is there any way to analyze this from 
a world systems perspective?  Can anyone recommend specific authors I might 
use?  As always, thanks for your input!

Jane Shahi