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Obama's power isn't as free floating as David depicts it here. True, Obama isn't dependent on any one faction, as our past several have been -- a strength of moving away from the "identity" politics the Democrats have used to organize themselves in the past. Instead, he built a (largely) online community, and brought together a network of progressive networks to form a new coalition.
But the idea he can now do anything he wants because the unions (or Wall Street, or feminists, etc.) don't own him is simply wrong. His power right now is based on the (accurate) perception that he represents a consensus, pointing in a generally progressive direction. If he violates that consensus, and becomes too hawkish or too corporate, or if he is seen as a tool of the establishment, his vaunted e-mail list will be of no use to him. If he goes against the reasons people signed up on MyObama.com, he'll have no army of constituents to activate to push legislation; they'll simply back away.
The laws of political thermodynamics still apply, even in a people-powered environment. Responding to the public interest is similar to satisfying a special interest, in that the client still needs to they're being attended to.