Obama is keeping his decision quiet, but his staff in Chicago and party activists are buzzing about Biden, in large part because he can address two of Obama's biggest weaknesses - his lack of experience, especially on world affairs, and his reluctance to attack his opponent.
Obama plans to appear with his newly selected running mate Saturday, with the pick announced via text message to supporters. Obama also is believed to be considering Govs. Kathleen Sebelius of Kansas and Tim Kaine of Virginia, and Sen. Evan Bayh of Indiana.
But Biden is at the center of much speculation now. Biden, 65, first was elected to represent Delaware in 1972. Obama was 11 at the time; half the people living in the U.S. were not born when Biden arrived on Capitol Hill. He is a curious front-runner to join a ticket headed by Obama, who prevailed during the primaries by making the case that he is an outsider who can bring change to Washington.
Biden has a compelling personal story: His wife and daughter were killed in a car accident a few weeks after he was first elected, but two sons survived serious injuries in the crash. Biden commuted home to Wilmington daily to care for them, a practice he continues to this day. The oldest son, Beau, is now Delaware's attorney general and a National Guard member whose unit is being deployed to Iraq in October.
Biden got another scare 10 years ago, when two brain aneurysms kept him out of the Senate for several months.
This week Biden returned from a trip to the former Soviet state of Georgia that he made at the invitation of the embattled country's president, a well-timed reminder of the value he could bring to Obama's ticket.
Fighting between Georgia and Russia has only increased the sense that Americans will turn to the candidate they believe will be a strong international leader.
Sen., the presumptive GOP nominee, brings a military background and a leading role on the Senate Armed Services Committee. Obama only has served three years in Washington, but Biden is chairman of the Senate Foreign Relations Committee.
Polls suggest the race between Obama and McCain is tightening, and Obama is responding by stepping up his attacks in speeches and commercials targeted to key states. Obama has never been entirely comfortable going negative, but Biden is always ready for a fight.
Obama could have been describing Biden when he said in a speech Tuesday that he wants his running mate to be "somebody who is mad right now" about the state of the economy, an independent spirit who will speak out when Obama's wrong and help him through major issues.
During the Democratic primary, when he also sought the presidential nomination, Biden often made the most memorable impression in debates even though he was barely registering in the polls. He got big laughs for accusing Republican candidate Rudy Giuliani of mentioning three things in every sentence - "a noun, a verb and 9/11" - and also leveled barbs at Obama, questioning his experience.
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He said he didn't think Obama was ready to be president yet, saying it's "not something that lends itself to on-the-job training." He offended some blacks when on the first day as an official presidential candidate he tried to compliment Obama as "the first mainstream African-American who is articulate and bright and clean."
Biden dropped out of the race after a poor showing in the Iowa caucuses.