Thanks for raising the question. I oten wondered about it. I quickly found this on the internet. (I don't know why "vice-president" is in the title -- irrelevant to the answer here):
http://wiki.answers.com/Q/What_happens_if_the_presidential_nominee_dies_before_picking_a_vice_presidential_running_mate

It would really be interesting if he died just days before the November election, which I guess would be postponed. Not enough time just days before Nov 4th to select someone, campaign, change ballots, etc.

Something like this happened recently -- when Eagleton was replaced as vice-presidential nominee shortly after he was nominated.

Robt.

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