Do not forget that in '32 the SPD backed Hindenburg… who then nominated Hitler as chancellor.
Thälmann was foolish, but even if he didn’t run, Hitler would still get into power. If the far right is strong enough, mere electoralism will not stop them. Fighting them must happen on the street level.
Do it very early before they’ve metastasized. Do it after they have power too. The pretext already exists, they campaigned on it. Being a partisan is now literally a fight for your life.