Let's say a mentally deranged loony (US assassins usually are) assassinates Cox in September 1920. FDR gets promoted to the top of the ticket. I doubt Harding is defeated but what would the effects be on both parties?
Would FDR as candidate for VP be automatically bumped to the N°1 spot? Did the Democrats have a provision for such a vacancy between the convention and the election? And who will pick the new candidate for VP?
As I understand it, the Convention can be recalled if time permits. Otherwise, the National Committee makes the choice, in a "weighted" vote with the member from each state casting the number of votes their state would have castr at a Convention.
It wouldn't have to be FDR, but he would be as likely as anyone. As for the VP, he would most likely to be another Midwesterner, if one could be found, or else a Southerner, maybe Underwood.
That's more or less what I thought. I remember reading a good novel by Fletcher Knebel, called "Dark Horse", in which the candidate dies a month before the election. The new candidate is chosen by the National Committee according to the procedure you describe. Incidentally, the candidate is not the candidate for vice-president, but a... dark horse!