Presidential Elections: Vacancies in Major- Party Candidacies and the Position of President-Elect [October 6, 2016] [open pdf - 671KB]
"What would happen in 2016 if a candidate for President or Vice President were to die or leave the ticket any time between the national party conventions and the November 8 election day? What would happen if this occurred during presidential transition, either between election day and the December 19, 2016, meeting of the electoral college; or between December 19 and the inauguration of the President and Vice President on January 20, 2017? Procedures to fill these vacancies differ depending on when they occur. […] Succession procedures during the presidential election campaign period and the transition are governed by party rules, federal law, and constitutional requirements. […] It may be assumed that party rules would be adequate to provide for some future incident, although an extremely volatile post-election political climate or the prospect of a faithless elector or electors might introduce complicating factors, as noted previously in this report. With respect to the transition period itself, the 20th and 25th Amendments have anticipated most potential contingencies, and could be implemented to address a succession issue during the period between the meetings of the electoral college and inauguration of the President."
CRS Report for Congress, R44648
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html