OTL, I've read where Woodrow Wilson nearly was seriously injured in a train accident during the 1912 Presidential campaign.
let's say he dies, but only after the election. So, what do the Democrats do?
If they want to ensure they get the Presidency, my guess is that they telegram electors and suggest that they vote for House Speaker Champ Clark, who was nearly the nominee but never quite got to the 2/3 majority. The reasoning being that they might be concerned about the election going to the House if enough people went for Roosevelt or Taft to make it a lot closer.
Would they see the wounded Wilson and decided, though, that it was better to promise another candidate just in case? Maybe advertise that Marshall was to be the nominee? Or, maybe they would remove Marshall from the ticket and put Clark on there in the hopes that a more quailfied Presidential candidate would rise to the position should Wilson die early in his term?
Another odd thought, though - how would Marshall doe as President if Wilson is elected and then dies of his wounds before the inauguration?