FACTBOX: The hows and whys of the vice presidency

(Reuters) - It may at times seem the least consequential high office in the world but the U.S. vice president is only a heartbeat away from one of the toughest jobs in the world.

While the spotlight is on presidential candidates Barack Obama and John McCain, speculation is rife over who they will select as running mates for approval at the Democratic and Republican conventions this month and next.

Here are a few facts about the job:

* Under the U.S. Constitution, the vice president has only two responsibilities. He is the presiding officer of the U.S. Senate but only votes if there is a tie. More importantly, he takes over if the president leaves office for any reason before his term ends. If that happens the new president nominates a candidate for vice president who must be approved by Congress.

* For most of U.S. history, the vice president did very little. Formal functions and state funerals were near the top of the list. John Nance Garner, a crusty Texan who served as vice president under President Franklin Roosevelt in the 1930s, famously said the job was not worth a bucket of warm spit.

But in the last quarter of the 20th century, vice presidents started having more input and played bigger roles in their administrations. Democrat Al Gore and Republican Dick Cheney are considered two of the most powerful vice presidents in history.

* Presidential candidates pick their running mates for a variety of reasons. Sometimes a liberal candidate will pick someone more conservative to gain ideological balance. Other times there might be a need to pick someone to ensure the support of a specific state or region, such as happened when John Kennedy of Massachusetts picked Lyndon Johnson of Texas in 1960.

An older candidate could pick a younger running mate to offset worries about age. Candidates who come from outside Washington often pick a Washington insider to get that balance. A less experienced politician might select a more seasoned running mate or the choice could be based on something as simple as the two candidates getting along and trusting each other.

* Nine vice presidents have risen to the top job because of death or resignation but only two -- Martin Van Buren in 1836 and George H.W. Bush in 1988 -- were elected directly from the office to the presidency.

* The vice president originally was supposed to be the person who finished second in the Electoral College state-by-state vote for president. That was changed after Thomas Jefferson and Aaron Burr tied in 1800 so that electors had to designate which job they were voting for.

* While Gore and Cheney put a brighter spotlight on the job and Obama and McCain are sure to make a big show of their selections this year, there is little evidence that who runs for vice president has a very big impact on voters' choices. Overwhelmingly, they vote for the presidential candidate, not the vice presidential one.

(Compiled by David Wiessler; Editing by Bill Trott)