Civil War Ceasar
Perhaps a dictator couold arise in the USA in the wake of the Civil War. Had Lincon had a different vice president, perhaps he could end up as dictator. The assassination could have been treated as a deep rooted plot, with extraordinary means needed to find and punish the culprit. Unlike in the extreme, but might be possible. Martial Law is used in several places, and things snowball.
Either that, or a Rebel general, by means of brilliant victories, saves the south temporarily, then is granted broad emergency powers. A cease fire is made, but he delays signing a formal peace--prolonging the need for emergency powers.