Speculation has been rampant for weeks among US media outlets, but there is still no consensus on who will get the nod. Obama has promised to let his supporters know first - via text message and email - and could reach a decision this week.
The choice of running mate is typically used to shore up perceived weaknesses in a candidate's qualifications for the top job, or to gain the White House hopeful additional support in a battleground state that both parties are competitive in.
Republican rival John McCain is likely to announce his pick only after the Democratic Party convention, which runs from August 25- 28 in Denver, Colorado. Media reports suggest McCain will make his announcement on August 29, three days before the start of the Republican Party convention in St Paul, Minnesota.
For Obama, who will be the first African American ever to be nominated by a major political party, there are a number of key voter concerns that may be allayed with his choice. A lawyer by trade who was elected to the US Senate in 2004, Obama lacks executive experience, as well as a strong background in economics and foreign policy.
None of these requirements narrows the Democratic field of possibilities. The top picks include - but are not limited to - the following names:
Hillary Clinton, 65, who lost to Obama in one of the tightest and longest Democratic nomination races in recent history. The so-called "Dream Ticket" would help unify the party and shore up support among working class, older and female voters that supported the former first lady. Still a polarizing figure in the Democratic Party, the likelihood of an Obama-Clinton ticket has dwindled since she suspended her campaign in June.
Joe Biden, 65, chair of the Senate Foreign Relations Committee and a former presidential candidate, would bring a wealth of foreign policy experience to the Democratic ticket. A party leader and Delaware senator since 1972, Biden could blunt Obama's anti- establishment message. His outspoken style and controversies that have followed could also detract attention from Obama before November.
Evan Bayh, 52, a popular senator and former governor of Indiana, has a reputation as a moderate, fiscally conservative Democrat. Serving on banking and armed services committees in the Senate, Bayh brings limited foreign policy credentials. But his executive experience and an economic background could help as the US flirts with a recession. His initial, vocal support for the war in Iraq could harm Obama's long-touted anti-war stance.
Tim Kaine, 50, governor of Virginia since 2006, could help Obama carry a state the Democrats have not won in 44 years. A rising star in the Democratic Party and co-chair of Obama's presidential campaign, Kaine is another Washington outsider that would solidify Obama's message of "change." Kaine brings little experience to the ticket outside of Virginia politics.
Bill Richardson, 60, governor of New Mexico and a former presidential candidate, would help shore up support among Hispanics and voters in some western states, but a Democratic ticket with two minority candidates is unlikely. Richardson has the broadest set of experience of any contender, serving as United Nations ambassador, energy secretary, congressman and two-term governor.
Kathleen Sebelius, 60, a governor of Kansas since 2003, brings an executive background and could help garner support among female voters that backed Clinton in the primaries. Sebelius could also help Obama win some Midwestern states, but has a low national profile. Other observers suggest that any woman candidate other than Clinton would only infuriate her erstwhile supporters.
Some other possible candidates for the ticket include Wesley Clark, a former NATO commander and 2004 presidential hopeful who would bolster Obama's military and foreign policy credentials.
Chris Dodd, a long-time Connecticut senator and leading economic voice in the party, has also been on the list. Dodd chairs the Senate Banking, Housing and Urban Affairs Committee and co-sponsored a massive housing rescue package passed by Congress this summer.
A long shot is Republican Senator Chuck Hagel of Nebraska, a vocal critic of the war in Iraq who accompanied Obama on his recent trip to the Middle East. Hagel would embody Obama's call for bipartisanship in US politics.