The enemy of my enemy is my friend. FDR was able to make America feel sympathy for the Soviets with the unprovoked attack of Barbarossa (similar to Pearl). We needed them to break a global threat.
I don't think FDR, or the other Warhawks, had to exert much effort to get the left and other anti Facists/nazis to feel for the Soviet citizens...
It was the isolationist portion of the middle-class, and the profacists among the wealthy who needed the full court press of persuasion.