If the president elect dies before being sworn in...

[Gary J]

As I understand it, the significance of taking the oath or affirmation prescribed in the constitution is not that it confers the office of President but that it is a requirement before the person who is  President can exercise the powers and carry out the duties of the office of President.

If you imagine a situation, similar to what happened to Ariel Sharon, where the President-elect was totally incapacitated by a stroke (so they could neither resign nor take up the duties of the Presidency) but was still alive; the President elect would become President at noon on 20th January.

Presumably in that situation the Vice President elect would have to be sworn in as Vice President and there would then have to be a short pause in the inauguration so the principal officers of the executive department (the outgoing President's cabinet) held a brief meeting to declare the President incapacitated, so as to allow the Vice President to become Acting President under the terms of the appropriate constitutional amendment.

A really difficult situation would be if the person who had won the Presidential election was to die on the day when the electors were due to meet and vote for a President and vice President elect. Particularly if the circumstances were such that some of the electors knew about the death when they voted.

There is precedent (Horace Greeley in 1872) that electoral votes cast for a person known to be dead are invalid.