So, in general, if the nominee dies in October, the other party's nominee is running unopposed? The VP-nominee doesn't come into play in any way? Interesting.

Replying late, but I just saw this.

I think some states allow the party to substitute in a new name if the candidate has died - at least if the ballots haven't been printed yet and/or if early voting hasn't started. But I wasn't really even thinking about whether you could switch the candidate, but whether a replacement candidate could possibly win. There's so many moving parts to a multi-state Presidential campaign, so much outreach and building relationships with different constituencies, that switching out the candidate with a month before the election will be so damaging that it's hard to see any campaign recovering.

That's if the candidate passes from natural causes. If the person were assassinated, that would create such an emotional reaction in the electorate that all bets would be off.