If it’s after they’re elected then it’ll run down the line of succession: VP, Speaker of the House, president pro tempore of the Senate, then the cabinet.
If it’s before they’re elected, I’m no expert but I believe the Democratic and Republican parties are private organizations that can do whatever the fuck they want, even fail to abide by the results of their own primaries.
If it’s after they’re elected then it’ll run down the line of succession: VP, Speaker of the House, president pro tempore of the Senate, then the cabinet.
If it’s before they’re elected, I’m no expert but I believe the Democratic and Republican parties are private organizations that can do whatever the fuck they want, even fail to abide by the results of their own primaries.