Complex sentence: These allegations were repudiated by supporters of the KPD: the right-wing leadership of the SPD, it was said, rejected the proposals of the KPD to unite for the defeat of fascism.
Split into simpler sentences: These allegations were repudiated by supporters of the KPD. The right-wing leadership of the SDP, it was said, rejected the proposals of the KPD to unite for the defeat of fascism.