I had this playful idea (concerning Against Gravity) of one of the main power groups being something called the US Government in Exile, a deposed dictatorship reduced to a small militant hardcore but armed to the teeth, off in some jungle somewhere. This would be within the context of a future US blitzed by irreversible ecological damage caused by industry and bad managent, as seems all too likely, eventually, in real life. Perhaps it would make more sense to say there's more than one US, rather like the US during the Civil War period? Have some gene-engineered disease wipe out half the US croplands, with various groups of States seceding from the Union to form their own separate political alliances. Make this group ultra-nationalist, right-wing, very militant, with an urge to reunite the other States by force rather than because they want to.
Don't laugh, look what happened to Russia. And yes, the Soviet satellite states weren't given much choice in the matter, but if things were bad enough ... after all, no Empire lives forever.