The Vice President
The primary responsibility of the Vice President of the United States is to be ready at a moment's notice to assume the Presidency if the President is unable to perform his duties. This can be because of the President's death, resignation, or temporary incapacitation, or if the Vice President and a majority of the Cabinet judge that the President is no longer able to discharge the duties of the presidency.
The Vice President is elected along with the President by the Electoral College — each elector casts one vote for President and another for Vice President. Before the ratification of the 12th Amendment in 1804, electors only voted for President, and the person who received the second greatest number of votes became Vice President.
The Vice President also serves as the President of the United States Senate, where he or she casts the deciding vote in the case of a tie. Except in the case of tiebreaking votes, the Vice President rarely actually presides over the Senate. Instead, the Senate selects one of their own members, usually junior members of the majority party, to preside over the Senate each day.