Since the time of Andrew Jackson’s run for the Presidency in 1828, individual political parties have had the job of filling any vacancy on their national ticket, either that of their Presidential or Vice Presidential candidate. If one of their candidates vacates the ticket after they are nominated, either because of death or withdrawal, the party selects a replacement.
The Democratic National Committee does, indeed, have rules in place to handle the situation. Article 2, Section 7 of the DNC Bylaws says that if there is a vacancy on the national ticket, a special meeting of the Committee ” shall be held on the call of the Chairperson,” where they would choose a new candidate. Such meetings make decisions based on a majority of those in attendance.
The same process would take place if the vacancy were to occur after the general election but before the Electoral College voting. 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.
Ultimately, it is the Electoral College (not the popular vote) that elects the President. The popular vote in the general election actually elects the states’ electors who form the Electoral College, which, in turn, elects the President and Vice President of the United States. These electors, chosen nowadays by state party organizations, meet in each state in mid-December following the election to cast their votes. No Constitutional provision or federal law requires electors to vote in accordance with the popular vote in their states, but the electors are made eligible to vote by being on the slate provided by the party that won the state’s popular vote.
Electors are generally committed to cast their votes for the winner of that popular vote, although some states do not require them by law to do so, and in some cases, Electors have refused to cast their vote for the Party nominee. Ultimately, the Electoral College votes are sent to Congress. The Congress meets in joint session in the House of Representatives to tally electoral votes on a date close to inauguration day. The President of the Senate certifies the outcome, and when that is done, the President and Vice President can be sworn in soon thereafter.
But what happens if President-elect has to withdraw after the Electoral College vote, but before Inauguration? Under those circumstances, Section 3 of the 20th Amendment kicks in, according to the Office of the Federal Register. The 20th Amendment says that in such a scenario, the Vice President-elect would become President.
— Kate Freeman