Here are some potential Obama VP running mate choices:
Evan Bayh: What he lacks in charisma, the telegenic Bayh makes up for in national security credentials, having served on both armed services and intelligence committees in the Senate.
Joseph Biden: A six-term senator who helms the Foreign Relations Committee, Biden could offer the heavyweight foreign policy experience that Obama is often accused of lacking. But at 65, and seen as part of the U.S. political furniture, he could undermine Obama's message of change.
Michael Bloomberg: Since ruling out his own independent bid for presidency, the mayor of New York has been seen as a potential running mate for both Obama and McCain. For Obama, the media tycoon and former Republican would help mitigate the Democrat's problem with Jewish voters brought on by rumors that he is a Muslim but do little to attract the white, working-class vote.
Wesley Clark: This former NATO commander, who failed in his bid for the 2004 presidential nomination, was seen as a staunch Hillary Clinton supporter, a fact that could help unite the party. But the 63-year-old's tough reputation as a no-nonsense soldier is unlikely to win much backing among party activists.
Hillary Clinton: Although the "dream ticket" of a Obama-Clinton campaign could help harness Clinton's power base of women and white working-class Democrats, the prospect of uniting the two rivals has won mixed support. A non-scientific CNN.com poll said 60 percent of people were not in favor of the move.
Chris Dodd: A long-serving senator with solid foreign policy credentials who was considered as a running mate for John Kerry's failed presidential bid in 2004, Dodd presents the same problems as Biden.
Charles Hagel: A close friend of fellow Republican John McCain, Obama's general election rival, Hagel's strong anti-war in Iraq stance has generated cross-party appeal, and though an unlikely choice, he could be seen as the man to attract wavering Republican voters.
Ed Rendell: As an outspoken Clinton supporter, Rendell could rally support for Obama, and as governor of swing state Pennsylvania, he could help secure key votes, but his popularity is limited outside Philadelphia.
Bill Richardson: The New Mexico governor, who identifies himself as Hispanic, could help sway the burgeoning Latino vote in addition to lending heavyweight foreign policy credentials as a former United Nations ambassador.
Kathleen Sebelius: The two-term governor of mainly Republican Kansas, Sebelius has proven cross-party support, but the rising Democratic star lacks a national profile.
Jim Webb: Another rising star, straight-talking Webb has dismissed his vice presidential prospects, but his appeal as a Vietnam veteran and successful novelist are clear. Webb's bluntness, however, led one commentator to label him an "unguided missile."