He picked a 35-year veteran of the Senate - the ultimate insider - rather than a candidate from outside Washington, such as Govs. Tim Kaine of Virginia or Kathleen Sebelius of Kansas; or from outside his party, such as Sen. Chuck Hagel of Nebraska; or from outside the mostly white male club of vice presidential candidates.
Hillary Rodham Clinton didn't even make his short list.
The picks say something profound about Obama: For all his self-confidence, the 47-year-old Illinois senator worried that he couldn't beat Republican John McCain without help from a seasoned politician willing to attack. The Biden selection is the next logistical step in an Obama campaign that has become more negative - a strategic decision that may be necessary but threatens to run counter to his image.
Democratic strategists, fretting over polls that showed McCain erasing Obama's lead this summer, welcomed the move. They, too, worried that Obama needed a more conventional - read: tougher - approach to McCain.
"You've got to hand it to the candidate and the campaign. They have a great sense of timing and tone and appropriateness. Six months ago, people said he wasn't tough enough on Hillary Clinton - he was being too passive - but he got it right at the right time," said Democratic strategist Jim Jordan. "He'll get it right again."
Indeed, Obama has begun to aggressively counter McCain's criticism with negative television ads and sharp retorts from the campaign trail.
A senior Obama adviser, speaking on condition of anonymity, said his boss has expressed impatience with what he calls a "reverence" inside his campaign for his message of change and new politics. In other words, Obama is willing - even eager - to risk what got him this far if it gets him to the White House.
Biden brings a lot to the table. An expert on national security, the Delaware senator voted in 2002 to authorize military intervention in Iraq but has since become a vocal critic of the conflict. He won praise for a plan for peace in Iraq that would divide the country along ethnic lines.
Chief sponsor of a sweeping anti-crime bill that passed in 1994, Biden could help inoculate Obama from GOP criticism that he's soft on crime - a charge his campaign fears will drive a wedge between white voters and the first black candidate with a serious shot at the White House.
So the question is whether Biden's depth counters Obama's inexperience - or highlights it?
After all, Biden is anything but a change agent, having been in office longer than half of all Americans have been alive. Longer than McCain.
And he talks too much.
On the same day he announced his second bid for the presidency, Biden found himself explaining why he had described Obama as "clean."
And there's the 2007 ABC interview in which Biden said he would stand by an earlier statement that Obama was not ready to serve as president.
It seems Obama is worried that some voters are starting to agree.
