I'd argue that political and cultural change most often follow technological change. Personally, I think we are on the verge of another technologically-driven upheaval, and that space exploration will only play a very small role, if any. I believe that our cultural systems are a reflection of social systems that are technologically possible and stable. Our situation will not be undone by political will, but by new technologies that make it obsolete. What we learn from space exploration can be a component, and I think it would be a positive one; I don't think all of them are.
In a word: decentralization. I believe that we are entering a new phase where the systems of nation-states are going to increasingly run counter to the realities of human interaction. In short, our technologies are going to enable global organization and exchange between people that will require new political structures. Probably something more fluid than any republic can offer. cc thundara