How did competition with the Communist Party of Germany affect the Social Democratic Party of Germany during the Weimar period?

During the Weimar period, the Social Democratic Party of Germany (SPD) was forced to expend significant effort competing with the Communist Party of Germany (KPD) for the support of the working class.1

This intense rivalry was a defining feature of the era, as both parties vied for the same political base. While the SPD maintained relatively stable core support among blue-collar workers, the ongoing competition with the KPD contributed to the broader political instability of the Weimar Republic. By 1933, this fractured political landscape was reflected in the Reichstag, where the SPD held 120 seats compared to the KPD's 81 and the Nazi Party's 288.1