The Process for Replacing a Presidential Nominee

The recent health episode concerning Hillary Clinton has highlighted the importance of understanding what would happen if either Clinton or Donald Trump should be forced out of the race between now and Election Day. The Clinton campaign has released a statement that Mrs. Clinton’s health scare was related to a previously undiagnosed condition of pneumonia and that she will be fine after a few days rest.
While both Hillary Clinton and Donald Trump appear to be in the best of health, there is little question that running for President is a highly strenuous activity. There always exists the possibility that one of the candidates could suffer a health-related event that would cause them to drop out of the race after receiving the nomination, but before Election Day.

No presidential candidate has ever dropped out after being nominated

In American history, no presidential candidate has ever dropped out of the race after receiving the nomination of his or her Party. However, one vice-presidential candidate died after being nominated (Horace Greeley – 1872), and another resigned from the ticket (Tom Eagleton – 1972). Both parties have a process in place to account for the possibility of their nominee having to drop out. According to here is what would happen:
Both the Republican and the Democratic parties have rules in their bylaws governing how to fill the vacancy. The Party Chair calls a meeting of the National Committee, and the Committee members at the meeting vote to fill the vacancy on the ticket. A candidate must receive a majority of the votes to win the party’s nod.
The same process would happen if the vacancy were to occur after the general election but before the Electoral College meets to vote. If a vacancy should occur on the winning ticket, it would then be the party’s responsibility to fill it and provide a candidate for whom their electors could vote.

Vice Presidential nominee has no special standing before Electoral College vote

Should Mrs. Clinton’s health issue require her to withdraw, the Democratic Party would have to decide if the current vice presidential candidate Sen. Tim Kaine would be the best choice to replace Mrs. Clinton. If not, they could pick another candidate to represent the Party. They are free to name anyone they wish.
If the winning candidate were to die after Election Day (November 8, 2016) but before the Electoral College meets some six weeks later (December 19, 2016), it would be up to the Party to name a replacement.
In other words, the vice presidential nominee on the ticket has no special standing should the presidential candidate be determined to be unable to run before the date the Electoral College meets (December 19) and would not automatically become the Party’s nominee. The Party would convene a special meeting and nominate a new presidential candidate.

Would be a major crisis for the nation

If a candidate should die or otherwise become unable to serve after December 19 when the Electors meet to record their votes, but before Inauguration Day (January 20), the 20th Amendment to the Constitution takes control and states that the vice president-elect assumes the presidency. However, there are murky waters here as well as there is some debate as to when a candidate becomes the “president-elect.” Most agree that it would be the date of the Electoral College certification (December 19), but some believe it would not be until January 6 when a joint session of Congress convenes to officially accept the vote of the Electoral College and declare the winner. This scenario would no doubt end up in the Supreme Court.
There is no question that the incapacitation of a presidential candidate between the time they are nominated by their Party and the certification of the Electoral vote by Congress would create a major crisis. It is surprising that the procedures are not more clearly laid out by law, and it may be time for Congress to establish a more formal process to account for this possibility before it becomes a reality.