Why does this matter more to me than all other considerations? We cannot build a majority so long as our standard abearers move to their right (or appear to in HRC's case) to win elections. We need leader who can move voters to the left! Feingold and Clark seem best suited for the job. All the other candidates, fine Democrats to be sure (not Biden), either can't sell it or are afraid to.
Also, I don't really think that Clark qualifies as "establishment" in anything like the sense that Clinton, Kerry, Biden et al do. I certainly don't see how he's any more "establishment" than Warner or Richardson, for example. To say the very least. Part of the establishment flirted with him briefly in 03-04 as the only apparent alternative to Dean, but they dropped him like a cold fish as soon as their boy Kerry surprised everyone by actually winning Iowa. Gore has also moved quite a ways away from the establishment over the past couple of years. I know that Clark, Gore and Feingold all appeal in different ways to many people who are dissatisfied with or infuriated by the current establishment, and while I can't see the numbers I would suspect that there is probably a significant correlation between support for these three here as well. That's a hypothesis based on substantial anecdotal evidence, anyway.