They liked Hitler until he invaded their beloved Soviet Union.
I don't think they ever liked Hitler. They took their talking points from Stalin's Comintern operatives. As long as the Molotov-Ribbentrop Pact was in force, and Nazi Germany was friendly with the Soviets, they were friendly to the Nazis. But that was always a marriage of convenience. They were always Soviet sympathizers first and foremost. Nazi Germany was too sympathetic to private enterprise for American leftists - their ideal was always state ownership of all productive enterprises and Nazi Germany was never going to be compatible with it.