As i understand it, instead of saying "we hate fascism—which is the inevitable result of capitalism etc and thats why while we support and endorse more aid to the ussr in their struggle we do not support American/Canadian/British/etc governments aims or policies in this war and must continue and deepen our struggle against our bourgeois and their fascist policies and will work with liberals (not parties, people) on this" the cpusa said "uncritical support for the USA and international liberal bourgeois states, the great allies of the USSR in the struggle against fascism."

The former position would prevent them from working under the liberals imo, as in, communists would still be a minority, but they would not be uncritically supporting their bourgeois states and opposing labour action.