What is the Electoral College and the 270-vote key?

U.S. voters don't directly elect their president and vice-president: instead, they choose through a constitutionally enshrined device called the Electoral College.

A vote for each candidate pair — president and vice-president run as a team — is actually a vote for the candidates' Electoral College voters.

Each party generally has a state-by-state list of Electoral College voters, but state laws vary on that point.

In each state, the number of electoral voters is equal to the number of the state's members of the House of Representatives and two for the state's senators.

In 48 out of 50 states, the Electoral College votes are determined on a winner-take-all basis: win the popular vote in the state, and you get all of the state's Electoral College votes.

The only states using a different allocation process are Nebraska and Maine, which use a variation on proportional representation. In those states, the winner of the total popular vote receives two Electoral College votes, and the winner of each congressional district receives one vote.

The Electoral College consists of 538 voters, and a majority of 270 votes is necessary to elect a president.

The number of votes allocated to each state is based on the 2010 census. The vote allocation will remain in place for the 2012, 2016 and 2020 elections.

After the election, the Electoral College voters meet in their respective states to cast their ballots for president and vice-president. The state results are then sent to U.S. Congress to be certified.