BEIJING, Feb. 21 (Xinhuanet) -- The following is an introduction of U.S. Electoral College:

Voters will only cast their ballots for a slate of electors in the U.S. Electoral College, who in turn directly elect the president and vice president.

The Electoral College, referring to the group of citizens selected to cast votes for the president and vice president, came about in the early 1800s and was first written into federal law in 1845.

The method of indirect elections was a compromise between those who wanted Congress to choose the president, and those who preferred a national popular vote.

Presidential electors are selected on a state-by-state basis. Each state uses its statewide popular vote on election day to appoint electors whose number is equal to the total number of both Senators and House Representatives in the state. The District of Columbia was allocated three electors.

Currently, the size of the Electoral College is 538-strong, but it is subjected to change with the census result.

Candidates for electors are nominated by their state political parties in the months leading up to election day, during primaries or at the party conventions. Under the Constitution, no person holding federal office, either elected or appointed, may become an elector.

On election day, all states and the District of Columbia choose electors by popular election. Although ballots list the names of the presidential candidates, voters actually choose electors for their state when they vote for the president and the vice president.

Most states, except Maine and Nebraska, employ the "winner-takes-all" system, meaning whichever ticket wins a plurality of voters in a certain state wins all of the state's electoral votes. Any pair of the presidential and vice presidential candidates who gains at least 270 electoral votes is claimed elected.

Electors chosen on election day meet in their state capitals orWashington, D.C. on the first Monday after the second Wednesday in December of election year, when they actually cast two separate ballots for the president and vice president.

If no presidential candidate wins at least 270 electoral votes, the selection is decided by the House of Representatives. If no vice presidential candidate receives a majority, the decision is left up to the Senate.

In the presidential elections in 1876, 1888 and 2000, the candidate receiving more popular votes nationwide did not win the presidency because he lost the contest for the electoral votes. The Electoral College thus came in for criticism because such outcomes do not logically follow the normal concept of how a democratic system should function.

Opponents claim the method diminishes the national popular vote's sway over the outcome of elections, causing candidates to attach more importance to large swing states with more electoral votes, in turn discouraging turnout and voters' participation.

In practice, the "winner-takes-all" manner of allocating a state's electors leaves third parties with very little possibility to break into the electoral politics that is dominated by the two major parties.