A few months ago, Mike defined the Compulsive Centrist Disorder and I have argued something similar a number of times in the past, e.g., here and here. In short – there is no such thing as a political middle. There is no line between “Left” and “Right” that you can put your own dot on. Most issues are quite binary – there is a “for” position and an “against” position, with each perhaps having additional modifiers. On each issue, one makes a decision. If on most issues you take a conservative option, you are predominantly conservative, and likewise for liberal option. Most people hold some of both. Rare are the people who are 100% one or the other.
The effective electoral strategy is to force the issues on which most people are taking your position and to prevent the opposition to force the issues that favour them. Nobody is wishy-washy middle (not even Joe Klein) – either you are liberal or conservative on any given issue. If Dems want to win, they need to remind people of their liberal stands.
There is an interesting article and discussion about this topic over on Rockridge Institute’s site called Biconceptualism that may be of interest to you (but read Mike’s and my definitions first).