I guess I overstated to make the point. Though the differences are much narrower at the top of the two-party system, you cite the paper to illustrate that the politicians still reflect the values of their conflicting constituencies and have to respond to some extent to their respective pressures. That's true. If the party representatives in the legislative and executive branches were politically indistinguishable and equally unresponsive to popular needs, as some contend, you wouldn't have a long history of the trade unions and the other social movements - including the newer ones - consistently supporting the Democrats. Instead, the Gomperist notion of "rewarding your friends and punishing your enemies" without distinction as to party would have much more currency than it presently does. It's the same in Europe as regards the labour/sd parties and the conservatives.
The main cause of left-wing disaffection from the mass leftish parties, of course, is that in administering the capitalist system they're necessarily required to mediate between the interests of their base and those of the large corporations. In a period when the masses are relatively quiescent, as at present, they are much less likely to take initiatives which disturb the status quo. The same distancing from the base is also true of the right-wing parties, where socially reactionary demands from below are shelved when they conflict with the needs of modernizing capitalism. There are reasons why the governing parties are usually described as being just left-of-centre or right-of-centre, and why they leave a trail of frustrated expectations in their wake.
[WS:] I think your analysis is right on the money, Marvin. There is a tendency toward political isomorphism in electoral politics, as claimed inter alia by Hotelling's law (http://en.wikipedia.org/wiki/Hotelling's_law ). This is especially true of the two-party system in the US, which can be though of as coalitions formed before the elections - which further emphasizes the compromise - whereas in a PR system coalitions are typically formed after the elections.
Of course popular understanding of systemic forces is close to nil. The "plain folks of the land" (to quote the memorable HL Mencken) prefer anthropomorphic explanations based on personal power, influences, ideologies, conspiracies etc, because that is what they can understand. I find it mildly amusing, however, when sophisticated people who certainly have a good grasp of systemic explanations fall for the same crap and blame Democrats for the alleged "betrayal" of the masses, while seemingly ignoring the systemic factors, such as the distribution of political preferences among the population and the balance of power.
Wojtek