What if a vacancy occurs after the primaries and before or during the convention?
If the leading candidate was to drop out of the campaign after most primaries or even during the convention, individual delegates would likely decide the party’s nominee on the convention floor.
What if a candidate left the race after the convention?
It would take a drastic event for a candidate to leave the race in the few months between a party’s nominating convention in the summer and the general election in November.
Democrats and Republicans have slightly different methods of dealing with this possibility. You can imagine the end result would probably be that the running mate stepped up to be on the general election ballot, but that is not necessarily guaranteed.
Democrats – The Democratic National Committee is empowered to fill a vacancy on the national ticket after the convention under party rules, after the party chair consults with Democratic governors and congressional leadership.
Republicans – If a vacancy occurs on the Republican side, the Republican National Committee can either reconvene the national convention or select a new candidate itself.