The Twelfth Amendment is one of the Constitutional amendments that will have a direct impact on the Presidential election of 2016.
Article II § 1 enumerates the process for electing the President and Vice President every four years. Simply put, the candidate with the most votes in the Electoral College would be President and the one with the second most votes would be Vice President; a tie would be sent to the House of Representatives along with the third top vote-getter.
However, in 1803, following the fight in the Electoral College for the Presidency between Thomas Jefferson and Aaron Burr, the Twelfth Amendment was ratified to avoid another stalemate. The election of 1800 was a race between Jefferson and then President John Adams. However, Jefferson and Burr received the same number of votes in the Electoral College, while Adams came in third. With a tie, the Constitution required that the House of Representatives vote according to one-state, one-vote. With support from Alexander Hamilton, the House vote swung to Jefferson as President; Burr received the next largest number of votes and became Vice President.
The Twelfth Amendment changed the process so that the Electoral College would vote specifically for a President and for a Vice President. In case of a tie, the House would elect the President from the three candidates with the most votes in the electoral college; the Senate would elect the Vice President from the top three candidates for Vice President.
The election of 1800 was significant for another reason: John Adams became the first President to be defeated for re-election. The election saw a peaceful transfer of power from the defeated to the newly elected, which has continued to this day.