The United States Electoral College is the institution that actually elects the President and Vice President of the United States every four years. The President and Vice President are not elected directly by the voters. Instead, they are elected indirectly by "electors" who are elected by popular vote on a state-by-state basis. Electors are appointed to each state. The number of electors in each state is equal to the number of members of Congress to which the state is entitled. The Twenty-third Amendment has always resulted in the District of Columbia having three electors. There are 538 electors, based on  435 representatives and 100 senators, plus the three electors from the District of Columbia. Electors are pledged to particular presidential and vice presidential candidates, though unpledged electors are possible, but unlikely. In all states, excluding Maine and Nebraska, electors are elected on a "winner-take-all" basis. That is, all electors pledged to the presidential candidate who wins the most votes in a state become electors for that state. Maine and Nebraska use the "congressional district method", selecting one elector within each congressional district by popular vote and selecting the remaining two electors by a statewide popular vote. Although no elector is required by federal law to honor a pledge, there have only been very few occasions when an elector voted contrary to a pledge.