The Vice President of the United States[1] is the first person in the presidential line of succession, becoming the new President of the United States upon the death, resignation, or removal of the president, should he or she accept the position. Every presidential term ends on January 20 of the year immediately after a presidential election. As designated by the Constitution of the United States,
the vice president also serves as the President of the Senate, and may break tie votes in that chamber.[2]
He or she may be assigned additional duties by the president but, as the Constitution assigns no executive powers to the vice president, in performing such duties he or she acts only as an agent of the president.
(bolding added)
http://en.wikipedia.org/wiki/Vice_Presiden...e_United_States
Origin of the position: Constitution (1787)
Article I, Section 2, of the Constitution appears to establish a succession to the Office of the President:
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The Vice President of the United States shall be President of the Senate, but shall have no vote, unless they be equally divided. The Senate shall choose their other officers, and also a President pro tempore, in the absence of the Vice President, or when he shall exercise the office of President.
Article II, Section 1 of the Constitution, in contrast, appears to establish the position of Acting President:
"In case of the removal of the President from office, or of his death, resignation, or inability to discharge the powers and duties of the said office, the same shall devolve upon the Vice President, and the Congress may by law provide for the case of removal, death, resignation or inability, both of the President and Vice President, declaring what officer shall then act as President, and such officer shall act accordingly, until the disability be removed, or a President shall be elected."
http://en.wikipedia.org/wiki/Acting_Presid...e_United_States