A chart from a new paper (pdf), “Why American Political Parties Can’t Get Beyond the Left-Right Divide”:
Seth Masket uses it to explain why moderatism has a small constituency:
As the circle in the center of the graph shows, there are some people who are moderate on both dimensions, but quite a few people are moderate on one dimension but extreme on another. That is, there are some people who are pretty non-committal about economic issues but feel very strongly about their social views.
How does a moderate political party rise to power with the help of moderate voters when so-called moderate voters are actually quite extreme on one dimension? It’s hard. If you don’t really care about changing abortion laws, that might make you appealing to social moderates, but many of those same people feel that taxes should either be much higher or much lower. You take the wrong stance, and you’ve just alienated half of them. Conversely, someone who’s moderate on economic issues might feel very passionate about their civil liberties.
It’s not impossible to be a true moderate on all the major issues of the day, but such people are rare, and the candidate that professes such beliefs will alienate more people than she wins over. In other words, the radical center never rises because, to a large extent, it doesn’t exist.