A social movement that could change the future of the United States (and possibly the world) is the Tea Party movement.
Since the Progressive Era of about 100 years ago, the United States has been moving more and more towards a situation in which the government plays a very important role in all aspects of our lives. It takes responsibility for paying our medical bills and our pensions as we get older. It regulates the sorts of advertisements we can see. It does many things in between.
If the Tea Party movement succeeds, this could change in a qualitative way. The US could move away from this sort of a society to one that was based on the idea of laissez-faire capitalism. This would constitute an incredibly significant change in the way we live our lives in this country.
The outcome affects the world obviously/