George Lakoff nails it on the head

[Lorien]

As cognitive psychologist George Lakoff told me: “Democrats moving to the middle is a double disaster that alienates the party’s progressive base while simultaneously sending a message to swing voters that the other side is where the good ideas are.” It unconsciously locks in the notion that the other side’s positions are worth moving toward, while your side’s positions are the ones to move away from. Plus every time you move to the center, the right just moves further to the right.

From: http://www.ariannaonline.com/columns/column.php?id=748