Manipulating public opinion turns democracy over on its head. Instead of leaders following the general population due to the opinions they hold, leaders convince the population to think certain ways. They can then create the opinions they choose to follow. This is a tactic commonly used in elections and by the political branches. Its what press conferences and media consultants are all about.
And its also what the public relations industry does. Click here for background on Edward Bernays, who is considered to be the father of public relations, and the subtle use of propaganda to entice the general public to arrive at preferred conclusions to public policy options. If successful, elected leaders can get the public to consent to things they might not otherwise consent to.
Here's a definition of public relations: the practice of managing the flow of information between an individual or an organization and the public.
Maybe the most famous current commentator who argues about the manipulation of public opinion - what he calls manufactured consent - is Noam Chomsky, but its a point made by many others.
More links to follow.