Consider this possibility in the not-too-distant future: Either John McCain or Barack Obama dies in office, or is forced to resign. Almost immediately, the United States would be led by either President Sarah Palin or President Joe Biden.

Slideshow: 9 Veeps Have Led U.S.

Using the past as a guide, it's a plausible scenario: Nine times in U.S. history, vice presidents, ready or not, have taken over as president -- eight of them after the man in the White House either died or was assassinated.

Being ready to take over as president is, in fact, one of the only official duties ascribed to the vice president by law. The other is serving as president of the Senate, a largely ceremonial role except when called upon to cast a tie-breaking vote over stalled legislation. The common role has been to shepherd the president's legislative agenda through Capitol Hill and function as a stand-in for the president in official capacities at home and abroad.

The post was so ill-defined that John Nance Garner, Franklin Roosevelt's vice president, once famously described it as being "not worth a bucket of warm (spit)."

Even today, as Vice President Dick Cheney wields a level of power and influence that many experts say is unprecedented, he only does so at the pleasure of President George W. Bush.

The issue of whether Biden or Palin is qualified to actually serve as president depends on one's point of view. Partisans have argued passionately for and against each vice presidential running mate this year.

Qualified or not, one of the two will occupy the vice president's residence after Inauguration -- and be a heartbeat away from the Oval Office. View this slideshow to see the nine vice presidents whose path they may have to tread.