So then, how could the KPD have received enough support to at least LOOK like they were going to win. That's all that is needed. Enough of what appears to be a momentum to scare Schleicher, the army and the conservatives.
Assuming Hitler drops dead
early in 1930. The NSDAP starts soon to fragment in a leadership struggle - so it will never see the surge to the leadership of the German right it got in the elections in September 1930.
But, for some reasons, this election isn't really held - Brüning has no majority, and doesn't wan't to rule by decree (why - dunno...). (Or - the election is held, but without a credible new force to the right, voter turnout remains low, the right remains extremely fragmented, and the SPD by far the largest party) So - Hindenburg and Brüning realize that a coalition against the SPD is difficult, and there is no real legitimation for rule by decree..
Anyhow - the SPD remains part of the government, but can't get additional spending pass - the economy is as shitty as OTL, so the SPD shares more of the blame... - This should facilitate the KPD line of reasoning about how Social-Democrats are just fascists with another label, at least somehow, and allow them to win more of the working class over..
comes along the 1932 Presidential election. Without the threath of a President Hitler, no one is really backing Hindenburg for a new term - there is a plethora of candidates, KPD, SPD, Zentrum, DNVP, and various groups of the right - will all run their own candidate for the first round.
Then, after the first round, KPD leader Ernst Thälmann has the strongest showing, getting 26 % of the vote. Now, the difficult task of finding a person that might unify the country against communism will arrise
- scenario a - this Person is found and wins round 2 with >70 % - and will start to rule a bit authoritarian due to his huge popular mandate...
- scenario b - this Person is NOT found - several people from the Zentrum and the right (at least 3) remain on the ballot - without a moderate socialist in the second round, the chances that part of the SPD voters might actually break for Thälmann seem real - so there is a military coup and the election is cancelled - or first results of the election show that Thälmann might win with a low plurality of 34 % - and then there is a coup, and the election result is supressed...
Although, in case of a coup, I'd expect the coup leaders not to call for really free and fair elections anytime soon - they are more likely to form a 'National coalition'-'Patriotic Front' - 'Movement for Germany' - or whatever and - if they feel the need to hold elections - allow a token opposition only - after a few years...
And, if it goes without a coup, the new President might actually become the leader of such a national rally, uniting the country against the communist threat...