Yes. Someday someone can write a long book on the misuse that has been made of two episodes of left history from 1920 through WW2: a) this matter of the KPD's failure to cooperate with the SPD and b) the campaign for a second front in the U.S. Every time anyone points out the difficulty in left campaigns which require implementation by a bourgeois bureaucracy someone gives as a couner-example the campaign for a Second Front -- a campaign which in fact had no impact on history since the U.S. waited until it was damn good and ready to launch a second front. The CPUSA had _no_ impact on that issue except in its own fevered imagination. (Similarly in Australia today the DSP has the wild fancy that it pressured the Australian state to intervene in East Timor, when in fact all it did was to give left cover to what Australia was going to do anyhow.)
And of course the point made above is definitive on the Hitler issue. As you say, for all its faults, the SPD _was_ a working class party; it makes one gag to imagine the DP being so considered. But also important is that the 1933 situation in Germany has _never_, in any way, been repeated. But what has been repeated over and over again is the case of allegedly "social democratic politicians" doing the dirty work of the capitalist class.
_If_ (or when) the threat of a U.S. Hitler arises, only a mass non-electoral movement (perhaps also engaging in electoral politics, but having its primary strength elsewhere) will be necessary to stop that rise. But the DP (unlike the SPD) exists for _no other purpose_ than to prevent the rise of mass politics.
I myself rather think that "Fascism" is now as defunct as "Bonapartism," and the next authoritarian menace will be unlike either. So looking backward at "fascism" is an aid to any future analogue to fascism. But in any case, the only U.S. politician of the last 50 years who might have been a figure like Mussolini was a Democrat -- Jerry Brown of Califorinia. The past _never_ reappears in recognizable form -- and guarding against past dangers is a sure way to open oneself up to disastrous surprise.
Carrol