Well. There was a failed revolution called the "Spartacist Uprising" in 1919. It was a serious threat to the new regime in Weimar and might well have succeeded if the government had no called in the right wing Freikorps (who attempted to seize power in 1920) to crush the uprising. However, I think that the coup was mostly confined to Berlin and was not given the chance to spread to other major industrial cities. The leaders of the coup, Rosa Luxembourg and Karl Liebnecht were both killed. Further, in the May 1924 Reichstag elections the Communist Party got 12.6% of the vote and 62 seats.