The urge to quote the Second Coming by William Butler Yeats is almost irresistible in this political environment. I say this, because Andrew Gelman has a great post about the limited usefulness of median voter theorem. Simply put, the median voter theorem is a political science theory that restates the conventional wisdom about how politicians must "run to the center" and accomodate the desires of centrist voters in order to gain and hold onto power. It's a comforting theory, because it assumes that the political system is self-correcting, stabilizing itself before sliding into extremism. Unfortunately, the reality is much different. According to the chart in Gelman's post (from Chapter 9 of Gelman's book Red State, Blue State, Rich State, Poor State: Why Americans Vote the Way They Do), adopting the position of the median voter gets you only about 2% of the vote in any election. The benefit of adopting the median voter position was higher in the '80s, but still wasn't any higher than 5% of the vote in Congressional elections.
Gelman doesn't point out that the benefit from adopting the centrist position has decreased continuously from the 1980s to the current decade. If I had to come up with an explanation, I would attribute it to increases in the importance of campaign contributions that give Congressional representatives the right to "buy" an extremist political position. (Votes, Money, and the Clinton Impeachment by Irwin Morris makes an extremely good case that Bill Clinton's impeachment occurred, despite large majorities of the public opposing it, because Republicans who supported the impeachment were more likely to get the campaign contributions they needed to survive the next election.) Even if a representative is way ideologically out of step with his or her constituents, a massive campaign warchest can go a long way in scaring off any challengers competent enough to run a campaign against an incumbent.
An Economic Theory of Democracy by Anthony Downs may also provide an explanation. In the model proposed by Downs, the Democratic and Republican parties are like two ice cream vendors competing for market share on the same street. Economic equilibrium can be achieved in two different ways. In one scenario, if you have a lot of people living in the center of the street, both the Democrats and Republicans locate their ice cream shop as close to the center of the street as possible. This would be the scenario predicted by the median voter model. In the other scenario, the Democrats stake out the left side of the street, while the Republicans stake out the right side of the street. This occurs when most of the population is concentrated on either the left side of the street or the right side of the street, with fewer people in the center. This distribution, called a bimodal distribution, is similar to what happens in American politics today.