If the Business Plot was real, and Im not saying it was or wasn't, it would have the most likely candidate to bring about Fascist America.
The US is so large to begin with that a fascist regime wouldn't need the same empire building as Germany and Italy to secure coal, oil, or agricultural self-sufficiency. It's hard to see an American claiming that Americans and Canadians are both one people in an analogue to Hitler's racial pan-Germanism, but Canada would probably just be Finlandized for cynical, geopolitical reasons.
A fascist US would look less like European fascism and more like a larger, English-speaking version of Latin American dictatorships or Juan Peron on steroids. Economic populism, nationalism, and extreme anticommunism can sell pretty well in the Americas, but territorial conquest really doesn't.