Because of the timing of this question, I assume it is being asked about the second edition of the interesting history book with the full title of The Art of Democracy: A Concise History of Popular Culture in the United States. Mentioning the edition is important because of two inclusions: a preface and a concluding chapter about computers and their impact. (The previous editions did not have that important component.)
In short, this book is about popular culture and its history of bringing minorities into the forefront of the United States public. Because it is a "history" of popular culture, it flows chronologically from the late 1700s on to the present day. Cullen begins with exploring the novels from the beginning of our country's history. Written word continues to prevail in popular culture until live theater takes hold, which allows minstrel shows to become prevalent in the mid 1800s. Cullen continues with the advent of movies, television, and computers. In each case, the new form of popular culture has to defeat initial resistance (often by the elderly population) in order to become mainstream.
In conclusion, it is important to note that in each case the popular culture mentioned allows the marginalized in society to be gradually accepted. There is no doubt that specific performers like Charlie Chaplin, Elvis, Billie Holiday, Jimi Hendrix, Culture Club, and Los Lobos rocket specific minorities into the mainstream of United States culture.