Theoretically, it is possible that churches, synagogues, and mosques on the Left + organized labor = a social democratic party equivalent in the USA, but they don't quite add up, because organized labor in the USA is weak and still wedded -- unlike (A), (B), (C), and (E) and probably much more so than even (D) -- to support for US imperialism at the level of the highest leadership (though not at the level of the rank and file, mid-ranking staffers, etc.).
[WS:] You are making an interesting point, somewhat similar to that made by Theda Skocpol in "Protecting Soldiers and Mothers." Skocpol gets one step further by arguing that the weakness of labor movement in the US can be explained by two factors: political openness of the US society in comparison to the 19th century Europe and political exclusion in the US running across class lines rather than coinciding with those lines as they did in Europe. The latter means that political exclusion in the US was mainly by gender, which put males of different socio-economic status in a privileged position vis a vis females of different socio-economic background. These divisions together with the ease to form associations created fragmented and localized movements that did not coalesce into a national class based labor movement as they did in Europe.
I would argue, however, that the proliferation of religious groups, many of them left and pro-labor leaning - is one of the main contributing factors to the weakness of organized labor. In fact, religion has always been used as a wet blanket for growing labor militancy - in Germany, Spain, Portugal, Italy, Netherlands, France - where "faith based" (mostly Catholic) labor organizations were deliberately set up to offset the growing popularity of labor unions and mutual associations. These faith based labor organizations embraced a watered-down pro-labor agenda, so they were progressive, but at the same time strongly discouraged militancy and were rabidly anti-socialist. Not surprisingly, they received support of fascist dictators from Mussolini, to Franco and Salazar.
I think these seemingly progressive religious groups played a similar role in the US less the openly fascist overtones - which was mainly a European specialite de la maison in the first half of the 20th century. While these organizations espoused a progressive agenda, they coupled that agenda with an ineffective organizational form (churches instead of industry wide unions or national parties). As a result, they were merely a "moral voice" without any political teeth. They offered mainly a symbolic action when a political action was needed, and a window of opportunity for such an action open. Consequently, they were one of the factors why the labor movement in this country missed the boat.
However, your main point is worth repeating - the absence of a national labor movement and a national labor party representing that movement is the main factor contributing to the right-wing drift of the US politics.
As to your comments on imperialism - I am not sure how that is relevant for social democracy and organized labor. I think adding the "imperialism" line to "socialism" was really a devious Soviet trick to enlist support of petty Third World nationalisms in their cold war with the West. In reality, "anti-imperialism" is a liability not an asset - it misleads the disgruntled Western lefties to embrace authoritarian, nationalistic regimes or dictators (Mao, Pol Pot, Castro, Arafat, to name a few) abroad as the means of defying the ruling class at home.
As a result, the anti-imperialism trope plays the same role as "pro-labor" religious organizations did. It moves social actions from the political and economic arena - where it really matters - into mostly symbolic arena, where it is mostly tough talking, guilt-tripping, and fist shaking. Quite frankly, when I hear 'anti-imperialism' I switch the channel, and I suggest that others do the same. It is not only a dead end, but a form of speech that antagonizes the very people whose support the left seeks - the working class.
Wojtek