President of the United States(4)

Election
Main article: United States presidential election



A map of the United States showing the number of electoral votes currently allocated to each state; 270 electoral votes are required for a majority out of 538 overall

Unlike most other countries using the presidential system, presidents are elected indirectly in the United States. A number of electors, collectively known as the United States Electoral College, select the president instead. Each state is allocated a number of electors, equal to the size of its delegation in both houses of Congress combined. Additionally, the Twenty-third Amendment to the Constitution grants electors to the District of Columbia as if it were a state, with the restriction that it may not have more representation than the least populated state. Electoral apportionment is adjusted every ten years, in alignment with the census. State legislatures are constitutionally empowered to appoint electors, however, all of the fifty states have established their popular selection.

History
Article Two of the Constitution originally established the method of presidential election. It also used an electoral college, but there was a major difference in the voting system. Each elector cast two votes, with the intention that one would be used for a presidential and the other for a vice presidential candidate. The candidate with the highest number of votes would become the president, with the second-place candidate becoming the vice president.
However, the 1796 and 1800 elections highlighted flaws in the electoral system in use at the time. In particular, the tie in the electoral vote that resulted from the lack of separation between presidential and vice presidential votes in the latter election was an issue. The Democratic-Republican Party's candidates, who won the election, were tied with each other, and as a result, the election was thrown to the House of Representatives in the outgoing Federalist Party-controlled 6th Congress. Federalist representatives attempted to elect Aaron Burr, the Democratic-Republican candidate for vice president, over Thomas Jefferson, the presidential candidate. Jefferson eventually won after Alexander Hamilton managed to swing one state delegation's vote to him. As a result, Congress proposed the Twelfth Amendment to the Constitution in 1803, and it was ratified in 1804. This amendment created the electoral system used today.

Campaign
Main articles: United States presidential primary, United States presidential nominating convention, and United States presidential election debates
The modern presidential campaign begins before the primary elections, which the two major political parties use to clear the field of candidates in advance of their national nominating conventions, where the most successful candidate is made the party's nominee for president. The party's presidential candidate chooses a vice presidential nominee and this choice is rubber-stamped by the convention. Also, the party establishes a platform on which to base its campaign. Although nominating conventions have a long history in the United States, their substantive importance in the political process has greatly diminished; however, they remain important as a way of energizing the parties for the general election and focusing public attention on the nominees.
Nominees participate in nationally televised debates, and while the debates are usually restricted to the Democratic and Republican nominees, third party candidates may be invited, such as Ross Perot in the 1992 debates. Nominees campaign across the country to explain their views, convince voters, and solicit contributions. Much of the modern electoral process is concerned with winning swing states through frequent visits and mass media advertising drives.

Source : http://en.wikipedia.org/wiki/President_of_the_United_States