Originally, the office of the vice president was a bit of an afterthought. One of the vice president's main responsibilities is to take over for the president should the president become incapacitated or die.
A role given in the Constitution to the vice president is president of the Senate, the person who presides over the Senate. The vice president is not given a vote in the Senate unless there is a tie. In that case the vice president may cast the deciding vote. Since the 1870s no vice president has cast more than 10 tie-breaking votes. The vice president has one other duty mandated by the Constitution, which is to open the certificates from the states that show the tally of electoral ballots cast for president and vice president in the presence of the Senate and House of Representatives.
Two of the constitutional amendments regarding the presidency also affect the vice president. The 12th Amendment stipulates that electors vote specifically for one person as president and one as vice president. In addition, that amendment states that the two people each elector votes for must come from different states. As a result, no presidential candidate can choose someone from the same state as a running mate. The 25th Amendment, which specified that the vice president succeeds to the presidency, gave the vice president some additional powers. First, it removed the possibility of the vice presidency being vacant by allowing the president to nominate a new vice president when a vacancy occurred, though the Senate had to confirm the nominee. When Spiro Agnew (vice president 1969–73) resigned after being indicted, Gerald Ford was nominated by Richard Nixon to be vice president and confirmed by the Senate. In 1974, after Nixon himself resigned and Ford became president, Ford named Nelson Rockefeller as his vice president. When Rockefeller was confirmed by the Senate, the two top federal offices were, for the first time, held by individuals who had not been elected to them.
For much of American history, vice presidential candidates were chosen to provide regional balance to the presidential ticket. The death in office of Franklin Roosevelt in 1945, the assassination of John F. Kennedy in 1963, and Nixon's resignation in 1974 meant that three of six vice presidents in fewer than 30 years succeeded to the office. That fact and the increasing complexity and power of the president prompted changes. As a result, several subsequent presidential candidates weighed their vice presidential candidates more carefully than had been the case in the past, focusing on choosing individuals they judged as experienced enough to serve as chief executive.
Another change that began around that time was to the vice president's role in the administration. For nearly two centuries, vice presidents were generally given little to do and were often kept in the dark on policy and ongoing issues. In 1977 Jimmy Carter began meeting regularly with his vice president, Walter Mondale, and gave Mondale specific issues to work on. Since that time, most vice presidents have taken on a greater role as an advisor to the president.