The Electoral College was established in Article II, Section I, of the United States Constitution and was later modified by the Twelfth and Twenty-third amendments which clarified the process.
When voters cast ballots for president and vice president, ballots show the names of the presidential and vice presidential candidates but voters are actually voting to elect a slate of "electors" that represent their state. The electors from every state combine to form the Electoral College.
Electors are also generally free agents, as only 29 states require electors to vote as they have pledged.
Each state is allocated a number of electors equal to the number of its U.S. Senators (always two) plus the number of its U.S. House representatives (which may change each decade according to the size of each state's population as determined in the census).
Each political party with a candidate on the ballot designates its own set of electors for each state, matching the number of electors they appoint with the number of electoral votes allotted to the state. This usually occurs at state party conventions. Electors are typically strong and loyal supporters of their political party, but can never be a U.S. Senator or Representative.
After the election, by statutes in 48 states and the District of Columbia, the party that wins the most votes in that state appoints all of the electors for that state. This is known as a "winner-take-all" or "unit rule" allocation of electors and it became the norm across the nation by the 1830's. Currently, the only exceptions to the unit rule are in Maine and Nebraska which allocate some of their electors by congressional district.
By federal statute, the electors for each state are required to cast their votes in mid-December, after which the votes are sealed and sent to the president of the U.S. Senate. Though the public votes for the party as a whole, the electors cast individual votes on separate ballots for president and vice president. This has become important in several elections in which electors voted for candidates other than those to whom they were pledged.
On January 6th, following the presidential election year, the president of the U.S. Senate opens all of the sealed envelopes containing the electoral votes and reads them aloud.
To be elected as president or vice president, a candidate must have an absolute majority, or greater than 50%, of the electoral votes for that position. However, a majority is not guaranteed within the Electoral College. An election with no Electoral College majority could occur in two ways:
1) if two candidates split the total of electoral votes evenly (with 538 electoral votes, a tie would mean a split of 269-269), or
2) if three or more candidates receive sufficient electoral votes to deny one candidate a majority.
If no presidential candidate obtains a majority of the electoral votes, the decision is deferred to the U.S. Congress. The House of Representatives selects the president, choosing among the top three candidates, and the Senate selects the vice president, choosing between the top two candidates. In the House selection, each state delegation may cast one vote and an absolute majority of states (26) is required to elect the president. (In this situation, Washington, D.C. would lose the voting power given to it by the 23rd Amendment since it does not have the same congressional representation given to the states).
However, a majority winner is not guaranteed in the Congress either. The states could feasibly split their votes equally between 2 candidates (25 state votes each) or the votes could be split between three candidates in such a way that no candidate receives a majority. Additionally, since every state only gets one vote, the representatives from each state must come to a decision on which candidate to support, potentially a challenge if a state has an equal number of representatives supporting two parties.
If a majority is not reached within the House by January 20, the day the president and vice president are sworn in, the newly-elected vice president serves as acting president until the House is able to make a decision. If the vice president has also not yet been selected, the sitting Speaker of the House serves as acting president until the Congress is able to make a decision. If a president has been selected but no vice president has been selected by January 20, the president then appoints the vice president, pending approval by Congress.
There have been only two amendments to the constitution involving the Electoral College. The Twelfth fixes a dangerous flaw, while the Twenty-third grants voting rights to the District of Columbia.
The 12th Amendment was adopted to fix a flaw in the Constitution that had allowed Thomas Jefferson to tie in the Ele3ctoral College with his vide presidential candidate Aaron Burr. The election was then sent to the House of Representatives which required 36 ballots to finally elect Jefferson president. The 12th Amendment specifies that electors shall cast distinct votes for the president and vice president, rather than electoral votes for two candidates. Full text of the 12th Amendment.
The 23rd Amendment gives Washington, D.C. representation in the Electoral College. It was passed by Congress in 1960 and was ratified in 1961 after being approved by three-fourths of the states. Full text of the 23rd Amendment.