We seem to have two different questions muddled up here:
1) Is there any difference (that makes a difference) between the two parties?
2) Has there been any change in the way the two parties operate in the last forty or fifty or whatever years?
My own answers would be 'no' and 'obviously' respectively.
> > I might have to concede that D's and R's are marketing themselves
> > to non-identical though extensively overlapping demographics within
> > the corporate/financial elites.
>
> But they can't win elections without winning popular support. The R's
> pitch to bigots and the petite bourgeoisie; D's to the working class.
> They've got to throw some red meat to these constituencies now & then.
Or seem to. Which is, I think, more what the Dems do.
"Popular support" is not a very precise concept. They have to get a few more votes than the other guy, at least sometimes. In the case of the Dems, this mostly happens either because the constituency is habitually Democratic, like my UWS nabe, or because the Republican has fucked up real bad and needs a holiday in opposition -- as was the case with the '06 midterms and seems likely to be the case in '08.
> At the elite level, there's overlap for sure, but there are also some
> sectors that are pretty partisan: extractive industries to Reps,
> Hollywood to Dems. Finance is way more Dem than retail or mfg.
An analogy I like is Guelphs and Ghibellines in mediaeval Florence. Yeah, the division was originally about something -- HR Emperor vs. Pope, roughly -- but soon degenerated into a mere rivalry of cliques. The contention was very bitter, though, and the sense of partisanship ran very high.
A more up-to-date analogy might be Crips vs. Bloods -- except that this is excessively disrespectful to the Crips and Bloods.