12th Amendment
The Electors shall meet in their respective states and vote by ballot for President and Vice-President, one of whom, at least, shall not be an inhabitant of the same state with themselves; they shall name in their ballots the person voted for as President, and in distinct ballots the person voted for as Vice-President, and they shall make distinct lists of all persons voted for as President, and of all persons voted for as Vice-President, and of the number of votes for each, which lists they shall sign and certify, and transmit sealed to the seat of the government of the United States, directed to the President of the Senate;–the President of the Senate shall, in the presence of the Senate and House of Representatives, open all the certificates and the votes shall then be counted;–The person having the greatest number of votes for President, shall be the President, if such number be a majority of the whole number of Electors appointed; and if no person have such majority, then from the persons having the highest numbers not exceeding three on the list of those voted for as President, the House of Representatives shall choose immediately, by ballot, the President. But in choosing the President, the votes shall be taken by states, the representation from each state having one vote; a quorum for this purpose shall consist of a member or members from two-thirds of the states, and a majority of all the states shall be necessary to a choice. [And if the House of Representatives shall not choose a President whenever the right of choice shall devolve upon them, before the fourth day of March next following, then the Vice-President shall act as President, as in case of the death or other constitutional disability of the President.–]The person having the greatest number of votes as Vice-President, shall be the Vice-President, if such number be a majority of the whole number of Electors appointed, and if no person have a majority, then from the two highest numbers on the list, the Senate shall choose the Vice-President; a quorum for the purpose shall consist of two-thirds of the whole number of Senators, and a majority of the whole number shall be necessary to a choice. But no person constitutionally ineligible to the office of President shall be eligible to that of Vice-President of the United States.
While the Twelfth Amendment did not change the composition of the Electoral College, it did change the process whereby a president and a vice president are elected. The new electoral process was first used for the 1804 election. Each presidential election since has been conducted under the terms of the Twelfth Amendment.
The Twelfth Amendment stipulates that each elector must cast distinct votes for president and vice president, instead of two votes for president. Additionally, electors may not vote for presidential and vice-presidential candidates who both reside in the elector's state—at least one of them must be an inhabitant of another state.
If no candidate for president has a majority of the total votes, the House of Representatives, voting by states and with the same quorum requirements as under the original procedure, chooses the president. The Twelfth Amendment requires the House to choose from the three highest receivers of electoral votes, compared to five under the original procedure.
The Twelfth Amendment requires a person to receive a majority of the electoral votes for vice president for that person to be elected vice president by the Electoral College. If no candidate for vice president has a majority of the total votes, the Senate, with each senator having one vote, chooses the vice president. The Twelfth Amendment requires the Senate to choose between the candidates with the "two highest numbers" of electoral votes. If multiple individuals are tied for second place, the Senate may consider them all. The Twelfth Amendment introduced a quorum requirement of two-thirds of the whole number of senators for the conduct of balloting. Furthermore, the Twelfth Amendment requires the Senate to choose a vice president by way of the affirmative votes of "a majority of the whole number" of senators.
To prevent deadlocks from keeping the nation leaderless, the Twelfth Amendment provided that if the House did not choose a president before March 4 (then the first day of a presidential term), the individual elected vice president would "act as President, as in the case of the death or other constitutional disability of the President". The Twelfth Amendment did not state for how long the vice president would act as president or if the House could still choose a president after March 4. Section 3 of the Twentieth Amendment, adopted in 1933, supersedes that provision of the Twelfth Amendment by changing the date upon which a new presidential term commences to January 20, clarifying that the vice president-elect would only "act as President" if the House has not chosen a president by January 20, and permitting Congress to statutorily provide "who shall then act as President, or the manner in which one who is to act shall be selected" if there is no president-elect or vice president-elect by January 20. It also clarifies that if there is no president-elect on January 20, whoever acts as president does so until a person "qualified" to occupy the presidency is elected to be president.