Very early on in this thread you wrote:
> As I keep saying, the political scientists who quantify these things
> say that the differences between the two parties - in their electoral
> base and their voting records in Congress - haven't been as wide as
> they are now in decades, and maybe ever. I'd love to see an
> intelligent refutation of this, but I haven't seen on yet.
This "differences ... in their electoral base" is the key, isn't it? That's why the idea of "not much difference" does not resonate with a huge number of people, for when they compare themselves to their backwater Republican cousin, they see a pretty big difference. In an environment where a "far left" mailing list (LBO-talk) has more than one contributor considering the height of a candidate (absent smileys), it is not surprising that the public has accepted the natural duopoly. It is a small jump to discard one's idealism in favour of pragmatism in a manner that does not soil the conscience. Every now and then I meet a liberal who tells me, "yeah, I used to read Chomsky too, when I was young".
IMHO, the truth is that liberalism/leftism/humanism/whatever is an improbable project (with or without the second law of thermodynamics). I am willing to concede to the Dem fanboys that Obama (or others) have "a good heart", but why on earth would he/they embark on an improbably project? Isn't it more pragmatic, in fact more effective, to eke out small victories here and there, and hope that the shifting winds will once again sway the competing instincts of the populace towards liberalism?
While one cannot ask much of the Dems, it is becoming futile to ask much even of the populace, as Zinn does with his reminder that we are citizens not politicians (http://www.zmag.org/content/showarticle.cfm? ItemID=12413). From mailing-lists to blogistan the effort is to wallow entirely in the probable i.e., the political, rather than attempt to create the climate for better instincts to express themselves. IMHO (sanctimonious though the below may sound!), this is more a reflection of the received culture of "success" that modern leftists are unable to reflect/introspect on -- a variation of the idea that __things are the measure of all men__.
--ravi