I'll recycle an old post of mine:
The Communists never came close to winning control of Germany, and the reason is *not* that Hitler got support that would otherwise have gone to the KPD. in fact the NSDAP attracted few voters of the sort who would otherwise favor the KPD. "Relatively few KPD voters switched to the Nazis, despite a popular stereotype. Workers were far less likely than middle-class elements to be members of the NSDAP or to vote for the party...The massive rise in the NSDAP vote between 1930 and 1932 left the combined SPD/KPD vote more or less solid, again suggesting that previously organised workers were more immune to Nazi propaganda than many other groups in German society."
https://books.google.com/books?id=deGGAgAAQBAJ&pg=PA25
Yabbut ISTM the question is not whether the Nazis won votes from the KPD, but whether the voters who abandoned the more centrist parties for Nazi radicalism in 1929-1933 might have gone Red instead. It was that bloc of voters that was decisive. They wanted Something Done about the Depression, something drastic. The Nazis offered that; so did the KPD; but not any of the other major parties. And the Nazi program, as displayed before 1933, had a lot of socialist flavor.
From 1928 to July 1932, the NSDAP gained 34.8% of the vote, while the KPD gained 3.8% (to 14.6%). The NSDAP then
lost 4.3% by November, while the KPD gained another 2.3%.
Suppose the NSDAP isn't there, and the KPD gains about half of the OTL new NSDAP voters (15%). That would put them at about 30%, well ahead of the SPD. There could still be a coalition to exclude them from power, as with them and the NSDAP OTL; but what if the KPD changed tacticts and agreed to a Popular Front coalition with the SPD?
This might play out like Spain - the army would accept the election result initially, but might decide to intervene. As in OTL Spain, the army rebellion might be partial, and the German army would be
much smaller relative to party militias. So the Reds and the anti-reds might control different areas and have to fight a war.
Who might intervene in this war? The USSR, but also possibly Poland, Italy, or France?
WI the rebellion ultimately failed in Germany but succeeded in Prussia, leaving Prussia as a sort of Taiwan?