Excerpts are from this link: http://www.askmehelpdesk.com/politics/what-if-presidential-candidate-dies-179908-2.html

here is what would happen if the nominee died
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Both parties allow for this possibilities in their rules .The national committee of either party would meet to select a replacement . This is found in Article III of the Democrat rules and in rule 9 of the Republican bylaws .

The most recent example of this was not due to death but resignation . In 1972 Democrat George McGovern selected Thomas Eagleton as Veep candidate. Eagleton resigned after it was revealed that he had been hospitalized in the past for mental strain. The Democrat national committee chose Sergeant Shriver as his replacement .

1912 VP James Sherman died after being nominated for a 2nd term. The RNC selected Nicholas Butler to replace him; but they did so after the general election.

If the candidate kicked it after the election but before the electors meet then they would proceed as above knowing that electors are not bound to vote for the designated candidate.

If the candidate died after the electors meet then the 20th amendment kicks in.( Read section 3)

Quote:
If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President. If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President elect shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified.

According to Rule 9 of the Republican party (Republican National Committee) only the Republican National Committee is authorized to fill "any and all vacancies which may occur by reason of death, declination, or otherwise" for the presidential candidate. The committee re-assembles. According to rule 8-B, they must give five days notice for this meeting. Committee members "representing any state shall be entitled to cast the same number of votes as said state was entitled to cast at the national convention." They continue until a new candidate is chosen with a majority of votes.

Democrats have a similar process (http://www.democrats.org/pdfs/charter.pdf). Article Two, Section 7(c) of the Democratic Party rules stipulate that "a special meeting to fill a vacancy on the National ticket shall be held on the call of the Chairperson, who shall set the date for such meeting in accordance with the procedural rules provided for in Article Two, Section 8(d) of these Bylaws." Article Three, Section 1(c) of the rules state only the Democratic National Committee may fill vacancies in the nominations for president and vice-president.

post election:
...the General Election in early November does not actually elect the next President. The Presidency is decided by the Electoral College not by popular vote. Therefore, legally, one does not become President-elect until after the Electoral College meets.

The Electors meet in their state capitals on the first first Monday after the second Wednesday in December to cast their ballots. The votes are "counted" during a joint session of the Congress on January 6th. That is when we actually have a President-elect. About 1/2 of the states have laws requiring the Elector's to cast their vote for whomever they have pledged to vote for. Therefore, they could have to legally vote for a dead person.