Much of it didn’t disappear until 1941.
Why 1941? Why not earlier?
Because a significant component of the American Left was taking its cues from Soviet Communists. And they were allied with Germany until 1941, at which point the Red Left got on board with stopping Hitler.
The fact that the USSR helped in the dismemberment of Poland in 1939 gets forgotten by most.
Another lost Leftist triumph in WWII was French Leftists supporting the Nazi invasion to the point of sabotaging supply and communication lines. Their rationale was Fascism is closer to Communism than Republicanism. They supported the Fascist takeover of France as a social progression towards communism. Many of them went to the camps under the Nazis.