Washington, Aug.16: Rival American presidential contenders Barack Obama and John McCain will be guided by their immediate requirements when deciding on their respective running mates.

With the Democratic convention beginning on August 25, Obama is almost certain to name his pick before next weekend.

At 47, he is a lowly state senator with no foreign policy experience, and probably needs a reassuring, safe pair of hands at his side.

The Democratic contenders, according to The Telegraph, are Joe Biden, Evan Bayh, Tim Kaine and Sam Nunn.

The Democratic vice-presidential nominee is due to speak at the convention in Denver on August 27.

McCain is likely to wait until after his Democrat rival unveils his choice.

At 72, he would be the oldest president ever elected to a first term - increasing the possibility that he might die in office, leaving his vice-president to take the helm.

Alex Castellanos, a veteran Republican strategist, says "McCain needs economic expertise and physical energy, while Obama needs experience and gravitas."

The Republican contenders are Tim Pawlenty, Mitt Romney, Joe Lieberman and Tom Ridge.

The Republican running mate chosen by Senator John McCain will take the stage at the Republican convention in St Paul, Minnesota, in early September.

Obama is said by aides to be considering three men most seriously: Senator Joe Biden of Delaware, Senator Evan Bayh of Indiana and Governor Tim Kaine of Virginia.

Biden, 65, as the long-serving chairman of the Senate Foreign Affairs Committee would give Obama the much-needed heft on the foreign policy front. He could perform the traditional running''s mate''s job of attack dog but is viewed as garrulous and prone to verbal gaffes.

Bayh, 52, is youthful and telegenic and is a former governor of Indiana, a conservative state that Obama would dearly love to make competitive. But he strongly supported the Iraq war, even being a co-chair of the neo-conservative Iraq Liberation Committee - though he states he has no recollection of this.

A more risky, though possibly more exciting, choice would be Kaine, 50, who could give Obama a dramatic boost in Virginia - a big target for the Democrat. Kaine is a former Roman Catholic missionary who speaks Spanish and used to be a civil rights lawyer.

The most obvious choice for McCain would be Pawlenty, 47, who hails form a potential swing state. A steadfast ally who prizes his loyalty very highly - he is appealing to social conservatives, many of whom regard the Arizona senator with a measure of suspicion.

Mitt Romney, 61, a self-made multi-millionaire and former management consultant, would bring executive experience and expertise in economics - an area that McCain has admitted is not his strong suit.

Either Ridge or Lieberman would be a remarkable choice for McCain because they support abortion rights - a litmus test for conservatives. But they would shake up a race in which Obama holds many advantages, though the polls remain close, and gain massive press attention.

Ridge is a fellow Vietnam who was also US Homeland Security Chief while Lieberman was Al Gore''s Democratic running mate in 2000. Since then, Lieberman, an Iraq hawk and one of McCain''s closest friends, has become an Independent. An Orthodox Jew, he would help in the swing state of Florida and appeal to centrist voters. (ANI)