Critics have described Cheney as being essentially 'a secret president' [File: EPA]

Delaware senator Joe Biden will no doubt be eager to do all he can to unsure that he takes up the post of vice-president after the US presidential election in November.

Indeed Biden will be hoping to wield considerable power in Barack Obama's White House, just as Dick Cheney does as part of the George Bush's administration.

Under Cheney and his predecessor Al Gore, the vice president has become the president's right hand man, valued for his Washington experience, policy input and ability to deliver change through Congress.

"Cheney has essentially been called a 'co-president', or by some critics 'a secret president'," Rosiland Jordin, Al Jazeera's correspondent in Washington, says.

"The president of vice-president has become much more powerful and certainly anyone who would be looking at taking the number two position on the ticket would certainly think: 'I might have a broader portfolio, I might have the opportunity to have a real impact on the administration policies."

But the role has not always been so valued.

'Insignificant office'

John Adams, the deputy to George Washington, the first US president, said that the nation had created "the most insignificant office that ever the invention of man contrived or his imagination conceived".

Later John Nance Garner, vice president to Franklin D. Roosevelt, famously described his job as "not worth a bucket of warm piss" - or "a pitcher of warm spit" if you believe some more squeamish sources.

In Adams' day, the vice presidency was automatically given to the man who came second in the presidential electoral college, but this arrangement soon become untenable in the 1900s with the emergence of opposing political parties.

Within the parties, the winner of the nomination has often selected his top rival as running mate to unite different factions.

Other considerations, such as appeal to different geographical regions of the US or social groups, as well as policy expertise, have also been paramount.

In 1956, Adalai Stevenson, the Democratic presidential nominee, allowed convention delegates to pick his running mate.  

Limited duties

The VP's official duties are limited.

He is president of the Senate and has the casting vote in the event of a tie in the 100-seat chamber. And of course, he steps up to the top job if the president dies or is incapacitated.

Beyond that, particularly before Gore and Cheney, the position often involved little more than ceremonial duties such as representing the nation at funerals of foreign dignitaries.

Nine holders of the office have succeeded unelected to the presidency. The last was Gerald Ford following the resignation in disgrace of Richard Nixon in 1974.

Four vice-president's have gone on to be elected president.

Source: Al Jazeera and agencies