Here is a thought experiment for you.
What if the true swing voters are not moderate at all? What if they have no real political thoughts and are neither for nor against anything? They can’t be moderate because that would mean having an opinion on events and policies. What if the 10% that crosses back and forth between voting for Republicans and Democrats does so ONLY based on the brand identity of the candidate? They vote for the guy they “like” - as in the phrase “I don’t know, I just like the guy.”
I think Howard Dean could have picked up that 10%, but that is water under the bridge. Who among the current Democrats has the best chance of picking up that 10%? Who has the “I just like the guy” vote right now? Note: This is not the "He can win" argument. The "he can win" argument gave us Kerry. This is the "he kicks ass" argument.