The offices of President and Vice-President are created in the United States Constitution in Article II. The President serves for a term of four years. The Electoral College choses the President and Vice-President. Originally, the Vice-President was the person receiving the second most votes for President, but this was changed by Amendment XII so that both offices would be filled by the candidates of one party.
The electors are selected according to methods approved by state legislatures, and throughout American history, this has been nearly always a "winner take all" process. It is possible for someone to be elected President with fewer popular votes than another person. This has happened three times in history: in the elections of 1824, 1876, and 2000. In a few additional cases, the President has received less than half of the popular vote. Abraham Lincoln received the lowest percentage of the popular vote ever when he won the election of 1860.
George Washington established a precedent that was not incorporated in the Constitution, to the effect that no one would serve more than two terms as President. In 1940, Franklin D. Roosevelt invoked the danger of world war as the reason that the nation should not change presidents at that time. He was re-elected to a fourth term in 1944. After the war, Amendment XXII restricted persons to no more than two full terms.
The Constitution is vague as to the exact powers which a Vice-President acquires upon succeeding a President who dies in office. The first instance came in 1840, when John Tyler succeeded William Henry Harrison. Many held that Tyler was merely the "Acting President," but Tyler assumed the full duties of the office and established a precedent that has been accepted ever since.
The President must be a Native Born Citizen. No provision is made for determining this fact. Questions have been raised about Barack Obama`s status, but the Supreme Court has not allowed them any legal standing.
The President can only be removed from office by impeachment by the House of Representatives, followed by trial and conviction by the United States Senate. Richard M. Nixon resigned rather than face impeachment that he was certain to lose. Two presidents, Andrew Johnson and Bill Clinton, have been impeached, tried, and acquitted.