The primary role of the vice president of the United States is to be prepared to take over the duties of the president, should the president be unable to perform his duties due to resignation, death or another incapacitation. The vice president is also president of the U.S. Senate.
As the president of the Senate, the vice president casts the deciding vote in the event of a tie. Unless a tie-breaking vote is necessary, the vice president does not generally preside over the Senate proceedings, although that is officially within his duties.
Prior to 1804, the Electoral College only voted for the president, and the candidate who received the second highest number of votes became the vice president. Since the 12th amendment was passed in 1804, electors now vote for both the president and vice president.
In the history of the United States, 13 vice presidents have become the president. Eight of those achieved that position due to the death of the current president. Four vice presidents, John Adams, Thomas Jefferson, Martin Van Buren and George Bush, were directly elected as president during a national election. Gerald Ford is the only vice president to take over because of the resignation of the president.