[WS:] I do not think this quite addresses my argument, so let me try to rephrase it. The DP-style social democrats is as far left as most of the US electorate will go, but it represents the slightly left-of the center point for most of the European electorate. In other words what passes for "socialism" in the US is a center-left position in Europe. There are many structural reasons for that, ranging from exceptionally well organized and class conscious business aristocracy to internal divisions within working/middle classes, and to the role that political patronage (aka "machine politics") played in tying labor interests to those of the business establishment.
I do take your point that most social democratic parties are pro-capitalist liberal parties favouring Keynesianism and, I may add, generous social programs. I also fully agree with your point that this, far from being "treachery" by the leaders, more or less represents the position of the "median constituent" of these parties. As you may recall, I argued a similar position some time ago on this list (summarized here http://wsokol.blogspot.com/2012/01/day-after-neoliberalism.html) - that structural changes in the composition of the workforce and the expansion of "technostructure" (Galbraith's term) reduced the popular appeal of the "old'left" ideologies in favour of liberal and libertarian ones.
Wojtek