while the actual distribution of views among people is more like this:
Even that complicated picture oversimplifies. Most people are in the center of the graph, and all of the viewpoints mentioned are divided by other issues--foreign policy, eminent domain, abortion, gay marriage--so we'd really need a 5 to 10 dimensional graph to respresent how our views are distributed. But let's stick with this one for now.
So here's the question: With so many different points of view, why do we have a two party system?
When it comes time for voting we wind up sorting people like this:
Everyone votes for the candidate closer to their own view. So no matter what you actually believe, voting winds up endorsing one of the two "regular" ideologies. The line-up isn't always the same. If the Republicans run someone closer to the Theocrat or Paleocon corner instead of a straight conservative, he'll lose the votes of libertarians while picking up those of socialists (ie, Catholics in hardline unions). But no matter how the line gets drawn we get two factions holding everyone in them.
So why don't we have a third party? It's been tried. Ross Perot took a good shot at making one in 1992. But his group quickly dwindled down to the same irrelevance as the Libertarian Party. Same thing happened to George Wallace and Strom Thurmond's Dixiecrat factions, and Teddy Roosevelt's Bull Moose party before that. A new party can take hold, such as Lincoln's Republicans, but that only happens when one of the old parties (Whigs in that case) goes away.
This is a function of our electoral system. Duverger's Law says that any "First past the post" system (where the candidate with the most votes wins the only seat) will naturally have only two parties. Candidates will compete for 51% of the vote and shape their platform to attrack the crucial "swing voters" in the middle. Voters will look at the candidates and want to avoid wasting their vote. If their favorite candidate isn't going to win they'll vote for their 2nd choice to prevent the one they like least from winning.
So in theory our elections should concentrate on pleasing the few percent of voters in the center. There's some of that--the 2000 bidding war for prescription drug insurance was all about swing voters--but we're getting lots of elected officials who are moving to the extremes.
The problem is we don't have a single-election system. The first hurdle is the primaries or an equivalent nomination process. This is totally dominated by the core voters on each side, the liberals and conservatives in the diagram above. People with other viewpoints are much less likely to win the internal contests, so they aren't committed enough to put in the work necessary to be part of the primaries. The centrists are even more removed--they usually couldn't even decide which party's primary to participate in. So instead of getting two candidates who are close to the center ideologically, we often have two extremists.
The farther apart the candidates are, the nastier the election will be. It's much worse to have an extremist of the other side in power than a moderate so partisans are more motivated to fight for their side. That also provides a motive to paint the opponent as more extreme than he is.
One side effect is that moderate voters get driven away by this nastiness. 1992 was one of the few presidential elections to have an increase in voter turnout because Perot offered an alternative. When it went back to the usual two options those voters didn't come back.
Gerrymandering makes all this ever worse, because it slants the elections to ensure whoever wins the primary wins the general election. Legislators wind up picking their voters instead of the other way around, so the situation gets frozen in place until it the voters change their minds drastically.
Is there a cure for all this? I think so, but that'll have to be a different post.