"Progressives should not make the same mistake that Ernst Thälmann made in 1932. The leader of the German Communist Party, Thälmann saw mainstream liberals as his enemies, and so the center and left never joined forces against the Nazis. Thälmann famously said that ‘some Nazi trees must not be allowed to overshadow a forest’ of social democrats, whom he sneeringly called ‘social fascists.’
After Adolf Hitler gained power in 1933, Thälmann was arrested. He was shot on Hitler’s orders in Buchenwald concentration camp in 1944."
I’m gonna assume you’re still talking about the Nazis since that was your original comment so let’s look at the reichstag breakdown of the election prior to Hitler being appointed Chancellor.
The Social Democrats won 121 seats in November 1932, the communists won 100 seats. The Social Democrats were socialists and the communists were communists. The nazis had 196 seats in the 1932 election. So if the socialists and communists had combined they would have had 221 seats which is more than 196. And those were leftist parties who were bickering. So if the leftists had combined they would have kept Hitler from being chancellor when he was appointed that in January 1933. But what about the centre party? Well, they had 70 seats and had a significant wing that was left and wanted to work with the social democrats. Now if we are conservative about it and say just 25 of those 70 were leftists, that would bring the 221 up to 246. And if the other 45 went to the nazis, which all of them never would because it was a big tent with diverse view points, that would have brought a nazi coalition to 241. So not as big of a majority but still a majority for leftists.
So yes, again, if the socialists, communists, and leftist wing of the centre party had combined their powers and hadn’t been bickering, hitler wouldn’t have been chancellor.
Basic source for the election results of November 1932. There’s more pages for the parties and stuff on there so go ahead and poke around.
The problem here isn’t “leftist parties bickering”, it is self-evidently “the SPD aligning themselves with liberalism and fascism”. It’s not like the KPD refused to form a majority with other parties.
As an aside, “socialist” and “communist” are generally interchangeable terms and the SPD were neither by conventional definitions, but were instead (being very charitable to them) what we would call DemSocs.