The U.S. vice presidential debate takes place Tuesday. It is an opportunity for current vice president Dick Cheney and Sen. John Edwards to state their views on the issues and persuade voters to vote for their candidates. The role of the vice president has changed a lot over the years, from largely ceremonial duties to working as a close advisor to the president.

Constitutionally, the Vice President has only two jobs: to cast the deciding vote in the United States Senate in the event of a tie and be sworn in as President should the President die or be removed from office. And for most of American history, that is all the vice president did.

Beginning in the early 1900's, presidents started inviting vice presidents to attend cabinet meetings and even fill in when they were unable to attend them.

Over time vice presidents picked up more duties - meeting with heads of state, and attending state funerals. In recent years, some vice presidents have been close advisors to the president. Political analyst Stephen Hess of the Brookings Institution in Washington D.C., says, vice presidents have become increasingly influential.

"So often that we've often said that with each vice president, that he's the most powerful vice president, and we've always been right," he said. "So that Al Gore, we said that about him when he was Vice President for Bill Clinton. And then now we say it about Dick Cheney. Dick Cheney has been given considerable responsibility by the president."

The presidential candidate chooses a running mate based on many factors - including the geographic area he or she is from, compatible views on the issues, and his or her appeal to voters.

But Larry Sabato, Founder and Director of the University of Virginia's Center for Politics, says vice presidents generally do not have much impact on voters.

" When people go in the ballot box they are voting for president. The vice presidents are hangers on. They may be important to governance but they are not very important to voters on Election Day," he said.

During the campaigns, the vice presidential candidates are vocal cheerleaders for their running mates, urging voters to choose their party's presidential ticket. They also can be used in the role of political attack dogs-- criticizing the other party's presidential candidate, doing the campaign's dirty work, while allowing their presidential candidate to appear more presidential.