I've been rereading America (The Book): A Citizen's Guide to Democracy Inaction by Jon Stewart, et. al., and here's a paragraph they wrote about our two party system:

Each party has a platform, a prix fixe menu of beliefs making up its worldview. The candidate can choose one of the two platforms, but remember - no substitutions. For example, do you support univeral health care? Then you must also want a ban on assault weapons. Pro-limited government? Congratulations, you are also anti-abortion. Luckily, all human opinion falls neatly into one of the two clearly defined camps. Thus, the two-party system elegantly reflects the bichromatic rainbow that is American politcal thought (Stewart, 108).