It blows my mind that we’re still discussing this after the German left took this approach with Hitler.

There is no “after fascists”. Fascists are the enemy that we unite against. Hitler wouldn’t have come into power if the leftists and moderates cooperated and had a healthy relationship. Even if some factions of the left/moderates want to play off fascists for power, there’s still plenty of people in the relevant group who dislike the fascists. Unite with them to take down the fascists and elements of their party who empower them.

We’ll get nowhere if we assume the entire group of leftists/moderates are fascist supporters. We need to ally.