WASHINGTON — When U.S. voters choose a presidential candidate, they get a vice president as part of the package. There are several key factors that go into picking a vice presidential candidate.

“I have selected this man to be my running mate because I want to change Washington and get America back on track,”2012 Republican Presidential contender Mitt Romney, unveiling his choice for vice president, Wisconsin Congressman Paul Ryan.

Together, they form what’s called “the November election ticket.”

Selecting the right running mate

U.S. voters mark their November election ballots for two people running together on a political party's White House ticket: one for president, the other, for vice president. Tradition holds that the presidential candidate chooses his own running mate.

So, what's the value of the vice presidential candidate?

"It's to boost election chances, where even a small advantage you might get could translate into a lot of electoral votes," said George Mason University's Robert Dudley.

Along with having voter appeal, the vice president also needs to know well the levers of power in the federal government. That is because most presidential candidates in recent decades have not been Washington “insiders.”

Insider's know-how

In 1980, Republican outsider Ronald Reagan chose a very experienced Washingtonian, former ambassador and CIA Director George Herbert Walker Bush, as his vice president.

In 1992, Democratic Arkansas Governor Bill Clinton chose insider Senator Al Gore.

Eight years later, Republican Texas Governor George W. Bush picked former Defense Secretary Dick Cheney.

In 2008, a relatively new-to-Washington Senator Barack Obama selected a seasoned Washington veteran, Senator Joe Biden. Obama and Biden remain the Democratic Party's White House ticket in 2012.

Attracting new demographics

The vice presidential slot has also been used to bridge generations, or to transition between them. When the senior George Bush ran for the White House in 1988, he chose a much younger Senator Dan Quayle as his running mate to appeal to the so-called "baby boom" generation born after the end of World War II. Both Clinton and Gore, incidentally, were baby boomers.

The 1984 Democratic team of Senator Walter Mondale and Congresswoman Geraldine Ferraro marked the first time a major party had ever nominated a woman.

In 2008, Republicans chose their first female V.P. candidate, Alaska Governor Sarah Palin.

Once the election is over, the new vice president’s official duty is to ceremonially preside over the Senate and cast a tie-breaking vote when needed. The vice president also needs to be ready to assume the presidency should death or some other reason vacate the Oval Office.