Labeling social democrats as “social fascists” doesn’t sound like mature political analysis. And the Nazi triumph in 1933 was a damning retrospective judgment on those who couldn’t tell the center left apart from the far right. As an oft-repeated legend has it, the German Communist Party (KPD) was too sectarian, too purist to ally with the Social Democrats (SPD) to defend democracy. With the Left failing to identify who its real enemy was, Adolf Hitler could destroy the labor movement almost without a fight.

For liberals, this divided resistance to Nazism serves as an enduring indictment of left-wing sectarianism. They plastered this parable across centrist outlets during Hillary Clinton’s campaign in 2016, arguing that Bernie Sanders was dividing the united front against Trumpian “fascism” simply by contesting the Democratic nomination. Their British counterparts also deployed this “lesson from history” against Jeremy Corbyn, accusing him of not fighting Brexit with sufficient vigor.

In the weeks after Joe Biden’s rise to become Democratic Party nominee this March, many pundits hastened to roll out the same narrative. For some, the disappointment of those who supported Bernie Sanders in the primary offered a perfect target for such criticism. We should not bang on about the “sins of the center left,” a piece in Prospect averred, but instead direct our fire against the real enemy. Any failure to cheer on the anti-fascist standard-bearer Biden would mean repeating the KPD’s “superhuman myopia” in the fight against Hitler.