Obama plans to appear with his newly selected running mate Saturday, with the pick announced via text message to supporters.
Obama also is widely thought to be considering Govs. Kathleen Sebelius of Kansas and Tim Kaine of Virginia, and Sen. Evan Bayh of Indiana.
Biden, 65, was first elected to represent Delaware in 1972, when Obama was 11 years old and half the people living in the United States today weren't born yet. He is a curious front-runner for running mate for a candidate who won the primary by arguing he would be a fresh outsider who could bring change to Washington.
Biden is a charismatic and hard-charging campaigner with 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 same crash. Biden commuted home to Wilmington from each day in the Senate to care for them, a practice he still 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.
Biden returned Monday from a trip to Georgia 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 a leader who will be a strong international leader. McCain brings a military background and leadership on the Senate Armed Services Committee. Obama only has served three years in Washington, but Biden is chairman of the Senate Foreign Relations Committee that he has served on for 33 years.
Also, polls show the race between Obama and GOP rival John McCain 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, a move that threatens his call for civility in politics, but Biden has never shied from a fight.
"He's passionate, he's articulate and he's persuasive," said Democratic consultant Steve McMahon, among those who consider Biden Obama's smartest pick. "I think he would do for Senator Obama what Lyndon Johnson did for John Kennedy. He's got serious foreign policy experience, a long and distinguished Senate resume and he is one of the most effective surrogates that Senator Obama has right now who can go toe-to-toe with any Republican on any issue at any time."
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 who will speak out when he's wrong and help him through major issues.
During the Democratic primary when he also ran for 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 for mentioning three things in every sentence - "a noun, a verb, and 9/11" - and also leveled barbs at Obama.
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." He dropped out of the race after a poor showing in Iowa.
Republicans would be sure to revive Biden's criticisms of Obama and already envision a line of attack that says Obama is so inexperienced he needs a running mate who has been in Washington longer than McCain.
Biden is famous for being able to talk at length - sometimes a mind-numbing length - on any topic, but he has enhanced his standing in the vice presidential race by avoiding discussion of it.
Obama's running mate contenders have been instructed to be mum - a trait that is not considered Biden's strong suit. But he has played by Obama's rules, denying that he was being vetted when he most likely was. He bluntly acknowledged he'd take the job if asked, while jokingly warning Obama might not want him.
"I made it clear to him and everybody else, I never worked for anybody in my life," he told reporters last month. "I got here when I was 29. I never had a boss. I don't know how I'd handle it."
He gave nothing away Wednesday, as reporters staked out his home in anticipation of the pick. The senator took a load of brush in the bed of a white Ford pickup truck to the dump. He returned about 2½ hours later, saying he was going to be working on his property throughout the day and would have no further comment.