There are a lot of problems with this book, and I don't mean the stylistic ones (such as Roth's hatred for the Jewish family).
He has Lindbergh making his successful campaign by flying around the country in the Spirit of St. Louis, like he did in 1928. Sorry, it was in the Smithsonian Institution by 1940. Somehow I can't see them letting him borrow it back.
Lindbergh sweeps the states, including the south. Sorry, the South by then was solidly Democratic and not particularly isolationist. One could say they didn't care much for the prospect of being occupied again.
President Lindbergh meets with Hitler in Iceland. Sorry, that was occupied by the British. I'm sure they would have just loved to have had Hitler come there.
President Lindbergh makes an agreement with the Japanese government for hands-off in China. Henry Luce would have all the Time in the world to make President Lindy's Life unpleasant for this betrayal of Brave Little China, and he wouldn't have been the only one.
President Lindbergh pulls an SA, making the Ku Klux Klan an auxiliary police. Sorry, by then, it was far past its glory days of the twenties. Moreover, he referrs to the "Grand Wizard" -- that was Bedford Forrest's title. The head of the Klan then was Imperial Wizard James A. Colescott.
President Lindbergh also names the "American Nazi Party" as another auxilary. Would it have killed Roth to explain why the German-American Bund would have done a thing like that, or why he wouldn't have been censured if not impeached for such an outrageous act?
And one wonders why Japan would even attack Pearl Harbor in this scenario. By 1943 (after the climax of the book) I would think they would have attained control of China; no need to suffer under an oil embargo.
There is indeed a paucity of background, so to speak. The America First Committee had a large and varied cast of characters; from hidebound reactionaries to outright socialists. There were those who argued for a country armed (defensively) to the teeth; there were those who argued that any armaments whatsoever were corrupting and harbingers of fascism. How would President Lindy put together a government from the likes of Barton K. Wheeler and Norman Thomas? Never mind the outright nutcases like John T. Flynn and Harry Elmer Barnes, the intellectual backbone of the movement. (With James Colescott as a power player, it somehow seems odd that people like Flynn and Barnes were not involved.) Then too, the Communists would have turned savagely on the isolationists after the invasion of the Soviet Union.
When Sinclair Lewis wrote It Can't Happen Here, he had an American fascist leader based mostly on Huey P. Long (and others), and fortunately for his plot Long had died by then. He also had his fascist being supported by a man based on Father Charles Coughlin, and had to have the original stepping aside. Roth doesn''t even go to that trouble in using the historical background.
(Roth seems to have it in for Louisville; he has Lindbergh <spoiler> and says that the airport was "five miles out of town". Bowman Field, the only airport then, was then at the city limits if not within them.)
I wasn't surprised when the book won the Sidewise Award.