Reply #5: Bonus answer: because we need a centrist to beat Bush?

[Woodstock]

The overriding objective for many is to beat Bush. What he's done to the country has created a sense of urgency. To get enough votes, we need someone who doesn't appear too far left, but we don't want to go too far right in the process. We need someone who is willing to compromise on the issues that are less important, but compromise from a practical standpoint (Dean) rather than to seemingly embrace the other side (Lieberman.)

To keep the base satisfied, we need someone who has a decent record on the things that are important to us (choice, environment, civil liberties, worker's rights, among others.) And we need someone who will strongly challenge the Bush Administration and the Republican congress on their foolish/extreme positions - someone who clearly identifies as a Democrat first and foremost, and plays up the difference between us and them. (The former may sound like Liberman, but not the latter. Dean sounds like both.)