In Lynne Olson's recent interview in the New York Times dealing with her book Those Angry Days she notes that isolationism was "finished" by late 1941 and that a "sea change" occurred in American public opinion in the two years leading up to Pearl Harbour.
She talks about the dominance of the isolationist lobby in 1939, about both liberals and conservatives at the time having powerful reasons for wanting to keep the United States out of the war, and about the importance and strength of women's groups in the isolationist lobby.
She says little about why all this faded through 1940 and 1941 except to mention a disastrous anti-semitic speech by leading isolationist Charles Lindbergh in September 1941 which badly backfired.
She does claim that key interventionist groups were lobbying the president and congress to come to Britain's aid but doesn't say what motivated them or how ordinary public opinion in the United States was swayed in favour of interventionism.
It's a given that Pearl Harbour itself transformed the landscape. I'm interested in the period prior to that.