Every four years, citizens of the United States go to the polls to cast their votes for a new president. But the rules of electing a president in November and the rules governing how American political parties nominate their candidates for the presidency differ in important respects. In the general election, voters in each state choose electors, who subsequently vote for the president. The candidate who receives the majority of the electors’ votes becomes president. Although there have been calls to abolish the Electoral College, Americans have largely relied on the same system since 1804—when the Twelfth Amendment altered important aspects of how the Electoral College works—to decide the outcome of their general presidential election. In contrast to the relatively stable rules governing the general election, the rules by which American political parties nominate their presidential candidates have changed dramatically over the past two hundred years.