The Democratic People's Republic of Tankiejerk

The SPD backed Hindenburg because the alternative was... literally Hitler, and the KPD was not interested in a compromise candidate. The SPD's choices were reduced to "Back Thalmann, whose chances at gaining the position even with SPD help were slim, who didn't mind if Hitler won, and himself was the catspaw of a regime that was, at that very moment, shipping millions to concentration camps and starving political enemies en masse"; try to back a solo candidate of their own, which would result in nothing but Hitler assuredly winning; or backing the conservative ghoul Hindenburg in an attempt to buy time.

The KPD had abandoned the united front approach with the SPD since '28. While not the cause of all of the rise of the far-right in German politics, it seems difficult to argue that the abandonment of the relatively successful united front strategy didn't feed electoral opportunities to the far-right, resulting in the situation wherein Hindenburg, the conservative candidate, was the only candidate other than Hitler in striking distance of electoral victory in '32.

The SPD had no shortage of mistakes or ghoulish decisions in the Weimar era. But "We decided to vote against Hitler" was not one of them, even if the subsequent total inability of the left opposition to form any sort of coherent resistance in the Reichstag led to 'pro-cooption' conservatives in Hindenburg's regime gaining the upper hand and giving Hitler a belated victory anyway.