in an early season episode of the west wing, the vice president is being pressured to resign because of a public scandal (alcoholism i think? dave?) in the end, president bartlett tells him he doesn't want him to resign. the reason: "because i could die."
the irony of the episode, as with so many west wing episodes, is that in the endless political maneuvering, the heart of the issue was forgotten. that being that the vice president could become the president in a blink of an eye.
and for this we have john tyler to thank. the constitution is very unclear about the process of succession in the case of the president's death. lucky for tyler, william henry harrison was gravely ill for about a month, so he had a long time to get ready to make his move. after the president's death, he went to the capital and announced that he would take office. the response of the federal government was basically, "um...are you sure?....is that what we're supposed to do? does anybody know?" and since john tyler was the only one who acted with any certainty, the office was his. it became known as the tyler precedent, and has been used 7 times since. (it was not until 1967 that an amendment to the constitution made official what had happened 8 times.)
8 times! 8 presidents have died in office! 4 by assassination and 4 by natural death. that's more than 1 in 6 presidents that die in office. that number seems high considering the apathy and mixed motives with which we continue to choose and judge vice presidential nominees. especially since the other precedent that john tyler set was that a vice president, who has no campaign promises breathing down his back, can kind of do whatever he wants.