I am assuming you mean history when you say "it." I am basing this on the fact that you tagged this with "history making." I am also basing it on the fact that you had it in the history section before someone moved it and edited the question.
History is, as Napoleon said, "a fable agreed to." In other words, it is just a story that manages to get the most people to agree to it. To me, competing visions of history are put forward by historians and journalists and such. The ones that fit best with our ideas of what our past should be like, win.
Of course history can be manipulated. We emphasize the things that fit with our vision of how the past should be. We try to ignore those things that do not fit with our image.
Once we have a given history that we have agreed to, we often base public policies on it. For example, since we in America believe that we came from independent pioneer types, we do not need a welfare state to take care of us -- we can do it ourselves. Arguments based on the idea that we are self-sufficient do well in the US because of this view of our past.
Public opinion originates from multiple sources based on what facts people have, what their emotional response is to any previous facts they've had, any previous experience they've had with the "messengers," how the facts will affect them, and how the facts are presented to them.
For example, over the past year this country has debated the health care reform issue. In my opinion, public opinion was manipulated severely during this time. Major politicians and broadcast journalists who were against the reforms used myths and distortions of facts to enrage the public against reform. They manipulated the message so that public opinion would sway the way they wanted.
The "public" does not get all its information from one source. With many sources of information available that may represent different political, social, and even religious views on the same topic, public opinion can be manipulated by those with the power, influence, and money to get their message across to the most people. I'll refer you to my own blog posts of last September where I shared my opinion on this topic. The second link is to a recent article on how this past week's passage of health care reform in the House is viewed differently by Long Islanders.