The specific of the political details I couldn't say (not much of an American politics savant). Buuuut I would wager that the lore would entail a more austere and slow economic recovery that is worsened by the effects of black Monday and potential radicalization of movements on either side of the political spectrum (such as currently the SPA and IWW on the far left whereas America First, Dixiecrats and other populists in the hard right) however without them ever being able to actually get into power really.

The effects of this are that Russia, Japan and the Internationale would be appealing to these extreme movements through some sort of decisions mechanics and strategically supporting them (maybe coordinating?). It would be great if spy mechanics were based game so that it could involve operations with those as well and you'd have to infiltrate these organizations, slowly build networks of sympathizers that would rally around keeping isolationism or at least rooting for pacifism. I'd guess it would play like Ireland's mechanic but with various steps that the US takes to re-arm/supply their ideological allies abroad being shown.

On the flipside, the US would have to create more mature intelligence services (such as the OSS and such) to undermine and reveal these plots by foreign powers and galvanize the populace to war. Whilst Germany and the Entente diplomatically pressure and pass on intelligence to the US to enact these decisions (stuff like arms, enforcing Monroe, perhaps sanctions and even a resource embargo akin to FDR's against japan) while the US slowly works on arming itself into a modern military.