For John, BLUF: More than you want to know, before you need to know it. Nothing to see here; just move along.
This overall paper from the Congressional Research Service, Mr Thomas H Neale, author, talks to the question of what happens if we have incapacitation of the President or Vice President or both. This is from 27 September 2004, but things don't change very fast in this area.
The part interesting at this point is what if it is a candidate, between the nominating conventions and the Inauguration. That portion of the report, Candidates, starts on page 11 of the PDF found at the link. But, here are the words:
Succession During Presidential Campaigns and Transitions. The related issue of succession during presidential campaigns and during the transition period between elections and the inauguration has been the subject of renewed interest since the terrorist attacks of September 11, 2001. The salient elements of this issue come into play only during elections when an incumbent President is retiring, or has been defeated, and the prospect of a transition between administrations looms, but uncertainties about succession arrangements during such a period have been cause for concern among some observers. Procedures governing these eventualities depend on when a vacancy would occur.UPDATE: And here is Intrepid Reporter Rob Eno (and former LRCC Member), today, on the issue from a GOP point of view.
Between Nomination and Election. This first contingency would occur if there were a vacancy in a major party ticket before the presidential election. This possibility has been traditionally covered by political party rules, with both the Democrats and Republicans providing for replacement by their national committees. For example, in 1972, the Democratic Party filled a vacancy when vice presidential nominee Senator Thomas Eagleton resigned at the end of July, and the Democratic National Committee met on August 8 of that year to nominate R. Sargent Shriver as the new vice presidential candidate.
Between the Election and the Meeting of the Electors. The second would occur in the event of a vacancy after the election, but before the electors meet to cast their votes in December. This contingency has been the subject of speculation and debate. Some commentators suggest that, the political parties, employing their rules providing for the filling of presidential and vice presidential vacancies, would designate a substitute nominee. The electors, who are predominantly party loyalists, would presumably vote for the substitute nominee. Given the unprecedented nature of such a situation, however, confusion, controversy, and a breakdown of party discipline among the members of the electoral college might also arise, leading to further disarray in what would already have become a problematical situation.
Between the Electoral College Vote and the Electoral Vote Count by Congress. A third contingency would occur if there were a vacancy in a presidential ticket during the period between the time when the electoral votes are cast (Monday after the second Wednesday in December) and when Congress counts and certifies the votes (January 6). The succession process for this contingency turns on when candidates who have received a majority of the electoral votes become President-elect and Vice President-elect. Some commentators doubt whether an official President- and Vice President-elect exist prior to the electoral votes being counted and announced by Congress on January 6, maintaining that this is a problematic contingency lacking clear constitutional or statutory direction. Others assert that once a majority of electoral votes has been cast for one ticket, then the recipients of these votes become the President- and Vice President-elect, notwithstanding the fact that the votes are not counted and certified until the following January 6. If so, then the succession procedures of the 20th Amendment, noted earlier in this report, would apply as soon as the electoral votes were cast; namely, if the President-elect dies, then the Vice President-elect becomes the President-elect. This point of view receives strong support from the language of the House committee report accompanying the 20th Amendment. Addressing the question of when there is a President elect, the report states:It will be noted that the committee uses the term “President elect” in its generally accepted sense, as meaning the person who has received the majority of electoral votes, or the person who has been chosen by the House of Representatives in the event that the election is thrown into the House. It is immaterial whether or not the votes have been counted, for the person becomes the President elect as soon as the votes are cast.Between the Electoral Vote Count and Inauguration. As noted previously, the 20th Amendment covers succession in the case of the President-elect, providing thatincaseofhisdeath,theVicePresident-electbecomesPresident-elect. Further, a Vice President-elect succeeding under these circumstances and subsequently inaugurated President would nominate a Vice President under provisions of the 25th Amendment. A major concern that has risen about this period since the terrorist attacks of September 11, 2001, centers the order of succession under the Succession Act of 1947. What might happen in the event of a mass terrorist attack during or shortly after the presidential inaugural? While there would be a President, Vice President, Speaker, and President Pro Tempore during this period, who would step forward in the event an attack removed these officials? This question takes on additional importance since the Cabinet, an important element in the order of succession, is generally in a state of transition at this time. The previous administration’s officers have generally resigned, while the incoming administration’s designees are usually in the midst of the confirmation process. It is not impossible to envision a situation in which not a single cabinet officer will have been confirmed by the Senate under these circumstances, thus raising the prospect of a de facto decapitation of the executive branch. This concern has led to several proposals in the 108th Congress.
Regards — Cliff