The Constitution assigns very little power to the acting vice president. His main duty is to serve as the presiding officer of the United States Senate.
In this role, he has the power to cast the deciding vote in case of a deadlock in the Senate chamber. This has happened: the deciding vote was cast by Al Gore, for example, after a deadlock on a gun-control bill during the presidency of Bill Clinton. The vice president may be assigned other duties by the president.
Because the Constitution had little to say about the vice presidency, precedent and tradition have been important. In the nineteenth century, the vice president was very weak. In 1842, John Tyler became the first to take over after the death of a president. Upon assuming the office of the presidency, he was called "His Accidency" by his political opponents. But his presidency was important because he moved into the White House and assumed full presidential powers. Had Tyler hesitated, he could have been viewed as a weak, interim president until the next election. Three other vice presidents became president later in the nineteenth century, and they also struggled to gain recognition as president. They also failed to win election as president in the next presidential election. In fact, Andrew Johnson, Abraham Lincoln's successor, was even impeached.
In the twentieth century, the office of vice president became more respected-both in that office and upon succession to the presidency. Theodore Roosevelt, Calvin Coolidge, and Harry Truman all won the respect of their political parties and reelection as president after first serving as vice president.
Today, the vice president might travel on presidential business. He is more involved in the White House and the executive branch than he once was.
In the nineteenth century, vice presidents were chosen by the political parties at the nominating conventions. They were expected to serve the party and its president. In the twentieth century, presidential nominees often chose them for various reasons. One reason is to gain an advantage in a presidential election. For example, a vice president might be chosen partly because he comes from an important state.
The role of the vice presidency has changed over time. In the nineteenth century, a vice president who became president was viewed as a mere caretaker. In the twentieth century, the vice president became more visible in his office, assertive when becoming president, and able to win the presidency in his own right.