I'm seeing at least four possibilities:
A: Roosevelt is assassinated before or shortly after Inauguration Day; Garner becomes President and manages to hold the country together for another four years but finally takes a bullet from either the far left or the far right; the new Secretary-of-State-become-president cracks down hard on whoever did it, whoever is suspected of doing it, and anyone he doesn't like; they retaliate and civil war breaks out.
B: Roosevelt manages to make it to 1937 but then is shot by a far-righter; Garner thinks it must be those damned Reds and cracks hard down on them, provoking general unrest, and the country spirals into a Russian Civil War analogue.
C: Roosevelt makes it to 1936, but drops Garner from his ticket in favor of someone further to the left; Garner doesn't like this and coups Roosevelt, managing to successfully depose him. He then declares that Roosevelt has resigned for reasons of health; the public sees through this and protests occur in the streets. A few of them turn violent, and Garner orders in the police to clear the streets. The protestors are cleared but not without a fight, causing casualties among the police forces; Garner declares that due to the unstable situation, he is suspending the 1936 elections until further notice. Mass protests ensue, and Garner orders in the army; eventually, an infantryman sympathising rather more with the protestors thinks, "Enough!" and turns on his commanding officer. Others follow his example, and the army fragments along political lines, the fragments accreting paramilitaries and domestic and foreign volunteers of all stripes, as civil war erupts.
D: Roosevelt drops Garner and Garner coups him as in C, but the coup fails and Garner flees Washington, links up with rightist paramilitaries, and slowly marshals his strength until he can pull a Franco and attempt a second, more successful coup; however, the army and large parts of the population remain opposed to his rule, and they contest it via force of arms, Spanish Civil War style.
A: Roosevelt is assassinated before or shortly after Inauguration Day; Garner becomes President and manages to hold the country together for another four years but finally takes a bullet from either the far left or the far right; the new Secretary-of-State-become-president cracks down hard on whoever did it, whoever is suspected of doing it, and anyone he doesn't like; they retaliate and civil war breaks out.
B: Roosevelt manages to make it to 1937 but then is shot by a far-righter; Garner thinks it must be those damned Reds and cracks hard down on them, provoking general unrest, and the country spirals into a Russian Civil War analogue.
C: Roosevelt makes it to 1936, but drops Garner from his ticket in favor of someone further to the left; Garner doesn't like this and coups Roosevelt, managing to successfully depose him. He then declares that Roosevelt has resigned for reasons of health; the public sees through this and protests occur in the streets. A few of them turn violent, and Garner orders in the police to clear the streets. The protestors are cleared but not without a fight, causing casualties among the police forces; Garner declares that due to the unstable situation, he is suspending the 1936 elections until further notice. Mass protests ensue, and Garner orders in the army; eventually, an infantryman sympathising rather more with the protestors thinks, "Enough!" and turns on his commanding officer. Others follow his example, and the army fragments along political lines, the fragments accreting paramilitaries and domestic and foreign volunteers of all stripes, as civil war erupts.
D: Roosevelt drops Garner and Garner coups him as in C, but the coup fails and Garner flees Washington, links up with rightist paramilitaries, and slowly marshals his strength until he can pull a Franco and attempt a second, more successful coup; however, the army and large parts of the population remain opposed to his rule, and they contest it via force of arms, Spanish Civil War style.
