In Weimar Germany, the moderate center left was the Social Democratic Party, who were a body of similar character to the Labour Party in England, and considered 'the socialists' by all and sundry. It was the Communist Party which assailed this from the left, in terms which would need much explanation to make sense of today, with the aim of discrediting the Social Democrats as a leftist body, so they could succeed to leadership of the working class entire. Communist thugs did indeed join Nazi thugs in attacks on Social Democrat rallies and election activities. It became Communist policy to see Hitler's rise as a 'two birds with one stone' sort of thing. The Nazis would destroy the Social Democrats; under Communist leadership the workers would rally to revolution at the first taste of Nazi rule; Germany would become Red Germany under Communist rule. It is a strategy only a trained ideologue could be fool enough to propose, let alone believe in. Things did not, as you suggest, work out so well, and certainly not for those active as Communists, who at the first taste of Nazi rule found themselves not at the head of a horde of revolutionary workers, but hunted, beaten, disposed of impromptu by ingenious murder squads, or tormented in concentration camps.
If one seeks to transpose this to modern idiom, one or two adjustments must be noted. Politics in Weimar were violent and armed. The Social Democrats had an 'armed wing' for protection and defense, just as did other parties. The street brawl was a routine part of politics. The strategy of destroying the center takes on a different tone when actual armed violence is out of the question, at least on the left. If anything, that strategy becomes more dreamlike, more clearly a fantasy indulged in without the slightest expectation of actually acceding to authority and responsibility. That the Communists were a disciplined and armed party will be urged against the identification being made between the 'lefter than thou' of our day and the Communists of Weimar, on ground of their sharing a similar aim --- wrecking a center left party and taking it over as a revolutionary force, albeit in our day a revolution expected to involve no violence, as is the present style. It is true enough these 'red rose' sort of people are no organized body, and that each one considers himself or herself a fiercely independent thinker who takes orders from no one. It is also true they have forsworn violence, and would not engage in it themselves.
"There is no man so lonely as a dissenter in a Non-Conformist meeting."
"So much of left-wing thought is a kind of playing with fire by people who don't even know that fire is hot."
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for those requiring context. 
