It is a question as old as humanity. There are many historical speeches with have moved people and influenced history. Today, rhetoric faces serious limits. Even, great speeches by Obama have had just a very modest impact on public opinion. This New Yorker article discusses the limits of persuasion in the modern US political system. In some cases, well-prepared and excellently delivered political speeches by Obama have worked against his political interests. For an elaborate argument see the New Yorker article.