Washington: Starting in 1792, US presidents and vice presidents have been elected quadrennially on the election day, the first Tuesday after the first Monday of November.
On Tuesday, voters will again cast their ballots to choose the next president between Democratic candidate Barack Obama and Republican hopeful Mitt Romney, and draw to an end the year-long election campaign of 2012.
US voters only cast their ballots for a slate of electors of the US Electoral College, who in turn elect the president and vice president, reported Xinhua.
The current US presidential election system, featuring the electoral college, was originally established by Article Two of the Constitution, as a result of a compromise between those who wanted Congress to choose the president, and those who preferred a national popular vote.
The whole process of the presidential election can be divided into four phases, namely the primary elections, nominating conventions, the presidential nominee campaign and the national vote.
During the nomination race, usually from January to June of an election year, primaries and caucuses are held in 50 states, the District of Columbia and all US territories to elect a presidential nominee for each major political party.
After securing the party nomination, each party's de facto presidential nominee chooses a vice presidential nominee to run with him or her. The pair receive their official nominations at their party's national conventions, which take place during the summer of election year.
Between the conventions and the Election Day, all parties' presidential candidates run their campaign nationally, holding rallies, broadcasting TV ads and giving interviews.
They have to repeatedly adjust and state their policies and stances toward all issues concerning voters at home and abroad, and face challenges from rivals. They also hold nationally televised debates, which could be influential to voters.
As hundreds of millions of US dollars are needed for the longtime and extensive presidential campaign, candidates have to exert themselves to collect political donations from the public throughout election year.
On election day, voters, generally, cast votes to select the candidate of their choice, but the ballot is actually voting to select the electors of a candidate.
Under the Constitution, each state is allocated a number of Electoral College electors equal to the number of its Senators and House Representatives in the US Congress. The District of Columbia is given three electors. No representative of the US territories is in the body.
Most states, excluding Maine and Nebraska, employ the "winner-takes-all" system, meaning whichever ticket wins the popular vote wins all of the state's electoral votes. Any pair of presidential and vice presidential candidates who gain at least 270 electoral votes of the total 538 are claimed the winners.
Although the president and vice president-elect can be yielded on the Election Day, the official voting for them by the Electoral College is held on the first Monday after the second Wednesday in December.
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.
The new president and vice president are supposed to be sworn in Jan 20. In case the House of Representatives has not chosen a president-elect by that date, the vice president-elect becomes the acting president until the floor makes the decision. If the vice president-elect is also not known by then, the sitting House Speaker becomes the acting president.
Election Day also witnesses the re-election of all House representatives and one third of senators as well as some governors and local government officials.
In addition to candidates from the Democratic and Republican parties, several small political parties also put their candidates' names on the tickets. However, mainly due to the "winner-takes-all" method of states awarding electoral votes, US electoral politics is still dominated by the two major parties.