Will Mr Obama's choice give him a boost in the polls?
Speculation is mounting that Democratic US presidential hopeful Barack Obama will announce his choice of running mate before the weekend.
Observers say Mr Obama wants to make his announcement before the Democrats' nominating convention which starts in Denver, Colorado on Monday.
Joe Biden, Tim Kaine and Evan Bayh have all been mentioned as possible choices.
The speculation comes as a Zogby poll suggests Mr Obama's rival John McCain has a five-point lead over Mr Obama.
However, a Quinnipiac poll published a day earlier indicated that Mr Obama held a five-point lead over Mr McCain.
Daily tracking polls from Rasmussen and Gallup suggest that Mr Obama retains a very narrow lead over his Republican rival.
The BBC's Justin Webb says Mr Obama could announce his decision as early as Thursday.
Mr Obama will be hoping the announcement will give him a big boost in the polls. Whoever he picks will campaign alongside Mr Obama and will become vice-president if Mr Obama wins the presidential election.
Speculation about the potential candidates has recently been focused on Delaware Senator Joe Biden, Virginia Governor Tim Kaine and Indiana Senator Evan Bayh.
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Senator Biden - who made a short-lived bid for the Democratic nomination earlier this year - chairs the Senate Foreign Relations Committee.
Observers say Mr Biden, with his working-class, Catholic background, could help Mr Obama win over crucial blue-collar voters in swing-states like Pennsylvania and Ohio.
Governor Kaine, who was the first major political figure outside Mr Obama's home state of Illinois to endorse him, could be an electoral asset to Mr Obama in the battleground state of Virginia, where he has served as governor since 2006.
And Senator Bayh - a high-profile backer of Mr Obama's former rival for the Democratic nomination Hillary Clinton - could help Mr Obama unite the party after the tough primary battle.
Mr McCain is also preparing to announce his choice of running-mate.
Observers say the presumptive Republican nominee is planning to make his decision public on 29 August, the day after the end of the Democratic convention.
Possible candidates include Mr McCain's former rival for the nomination Mitt Romney, Minnesota Governor Tim Pawlenty, former Homeland Security Secretary Tom Ridge and former Ohio congressman Rob Portman.