Of any outcome that had a reasonable chance in Weimar, the victory of the SPD/Iron Front would have been the most optimal. Ideally, there would be an umbrella alliance for all anti-authoritarians, i.e. everyone who rejects Trumpism, Neoconservatism, Neoliberalism, Fascism, and Communism.
“To my conservative friends: let me introduce you to the Iron Front, the paramilitary wing of liberal democracy in Germany that fought both Nazis and authoritarian Communists (and monarchists, who are now irrelevant) in the lead up to World War II. They fought for the German republic and against all totalitarian ideologies. I invite you to pick up their mantel, and drive the fascists from your ranks. Proud Boys, American Front and Atomwaffen are all good starts on the national level. At the local level you can find various right wing gangs and protest groups that collaborate with these national organizations. Here in Oregon we have Patriot Prayer, Sandy Backs the Blue, and a recent visit from American Wolf out of Olympia. In your locality you are likely to find sympathizers and organizers who are allowing fascists to creep into your political party from the extreme right. Just like totalitarian strains of communism are creeping in on the left. Together we can end authoritarianism of all kinds!” –Vince Rinehart
The Iron Front (German: Eiserne Front) was a German paramilitary organization in the Weimar Republic that consisted of social democrats, trade unionists, and liberals. Its main goal was to defend liberal democracy against totalitarian ideologies on the far right and left, and it chiefly opposed the Nazi Party with their Sturmabteilung wing and the Communist Party of Germany with their Antifaschistische Aktion wing.
Formally independent, it was intimately associated with the Social Democratic Party of Germany (SPD). The Three Arrows, originally conceived for the Iron Front, became a well known social democrat symbol representing resistance against Nazism, Communism and Monarchism during the parliamentary elections in 1932, and was adopted by the SPD itself.