For a decade prior to 1933, the KPD called the SPD "social fascists" and lumped the SPD together with the rest of the political parties all as a bunch of fascists. The KPD was lead by a man named Ernst Thälmann and supported by the USSR. He had personally met Lenin at the Third International and followed Comintern policy - which by 1925 was really Stalinist and basically meant anyone who wasn't communist was considered facist. Moscow had control over many foreign communist parties, so their direction came directly from the USSR. Unfortunately for Germany, this meant the left wasn't able to try and unite until it was too late.

By 1932, the KPD and SPD didn't have enough control in the Reichstag to control the government even if they wanted to work together. Instead the Nazis formed a coalition government with other right wing parties and most importantly the Centre party which resulted in Hitler becoming Chancellor in January 1933. By this time, Thälmann saw what was coming and was living in a safe house he thought the Nazis didn't know about. He did make efforts to try and get some sort of resistance going and at that point made an effort to reach out to the SPD about a sort of general strike, but it was too little, too late.

At the end of February 1933, the Reichstag fire happened. Communists were made to blame, the Reichstag fire decree happened, and the Hitler had Thälmann and other leaders of the SPD and KPD arrested. In the March 1933 election, the KPD had been outlawed and the SPD was barely hanging on. By the end of March 1933, the enabling acts were passed (with the SPD being the sole votes against) which gave Hitler absolute power. After that, it was simply too late. The Nazis imprisoned many of the remaining SPD leaders, and then some SPD leaders started to fall in line with the Nazis to avoid imprisonment. The Nazi government first and foremost oppressed communists, socialists, and anything else that leaned left - by 1934 the Night of the Long Knives happened and anyone inside the Nazi party that leaned left were also "purged."

To go back a bit, the July 1932 election was considered the most violent ever in Germany. Just like the Nazis had the SA aka their "brown shirts" - the KPD had their red front - though it was officially banned in 1929, there was still an underground paramilitary wing of the KPD. The two sides clashed. But even throughout 1932, the KPD kept fighting against the "social fascists" in the SPD.

By 1935, the Comintern would change its view to being strictly anti-facist and proclaimed that communist groups in other countries should join together with any other anti-facist parties to stop the spread of fascism. This had a noticeable impact on France, which lead to the communists, socialists, and center parties working together to oppose right wing fascist parties, known as the "Popular Front."