Although officials told major media organizations of the decision hours before the long-awaited text messages were sent, none did so on the record, not wanting to pre-empt Obama's promise to give supporters the official running mate word first.
"Barack has chosen Senator Joe Biden to be our VP nominee. Watch the first Obama-Biden rally live at 3p.m. ET on www.BarackObama.com. Spread the word!" the text message read. The brief mobile alert was sent to most supporters just after 3 a.m. ET.
Biden, who himself made a bid for the nomination, joined Obama at a major rally in Springfield, Ill. Saturday, kicking off the first of a series of events leading to the Democratic National Convention in Denver next week.
At the rally, Obama introduced Biden to the crowd of thousands as his running mate, calling him a "leader who is ready to step in and be president."
Obama said Biden was "what many others pretend to be -- a statesman with sound judgment who doesn't have to hide behind bluster to keep America strong," according to the Associated Press.
Biden quickly went on the offensive during his remarks at the rally, attacking GOP rival Sen. John McCain and linking him to President Bush.
"We cannot as a nation stand four more years of this," Biden said. "You can't change America when you supported George Bush's policies 95 percent of the time."
Reaction to the pick emerged throughout the day from Biden's Senate colleagues and Obama's former rival for the nomination -- and rumored running mate possiblity -- Sen. Hillary Clinton.
"Sen. Biden will be a purposeful and dynamic vice president who will help Sen. Obama both win the presidency and govern this great country," Clinton said in a statement.
Even some GOP senators chimed in with their praise for Biden as the pick.
"I congratulate Sen. Barack Obama on his selection of my friend, Sen. Joe Biden, to be his vice-presidential running mate. I have enjoyed for many years the opportunity to work with Joe Biden to bring strong bipartisan support to United States foreign policy," said Sen. Richard Lugar, R-Ind., in a statement.
Word finally leaked of the selection shortly after midnight Saturday, after reports and rumors about the nomination rattled around the Internet and cable news outlets.
First the name of U.S. Rep. Chet Edwards of Texas emerged as a possible contender during the day Friday, unexpectedly joining a list that already included Biden, Virginia Gov. Tim Kaine, Indiana Sen. Evan Bayh and Kansas Gov. Kathleen Sebelius.
But the primetime news came and went without any announcement. Then, late in the evening, word came out that both Kaine and Bayh had been informed that they were not the running mate pick.
Finally around 12:48 a.m. ET the first Associated Press wire story moved citing a Democratic official confirming Biden as the nominee.
During his unsuccessful run for president, Biden had gained some traction with voters by stressing the need to thoughtful leadership on foreign policy matters, an area in which Obama has far less experience.
Speaking to supporters just before the Iowa Caucuses he said, "I want all the caucus-goers in this great state to close their eyes and imagine: If their candidate is president of the United States, not in a year but this very instant, are they confident that they have the sure-footedness, the steady enough hand to know exactly what they would do in Pakistan? To know exactly what they would do -- not generically -- exactly what they would do in Iraq? Are they ready? Because, ladies and gentlemen, they are going to have to act."
This ability to deal with foreign policy issues was on display earlier this week when Biden was invited by the embattled president of Georgia, Mikheil Saakashvili, to visit Georgia in the midst of its crisis with Russia.
The Biden selection was widely seen as a move to bolster Obama's limited foreign policy and national security experience and no sooner had the unofficial word leaked, than the McCain camp was using the pick to question the Ill. senator's readiness to lead.
McCain spokesman Ben Porritt said in a statement that Biden had "denounced Barack Obama's poor foreign policy judgment and has strongly argued in his own words what Americans are quickly realizing -- that Barack Obama is not ready to be president."
The McCain camp also swiftly released video of Biden criticizing Obama during early Democratic primary debates.
Biden ended his bid for the Democratic nomination in after a disappointing fifth-place finish in Iowa, eventually coming out for Obama over his other Senate colleague Hillary Clinton.
Although he had earlier told FOX News he had no interest in the number two slot, Biden had said on the NewsHour in November 2007 how he had been struck by the personal desire he felt to lead the country and tackle the issues facing the country.
"[W]hen I started, I wondered whether I'd have the passion and the energy to do this. What has surprised me, I feel more passionate about the possibilities for this country than I did when I was 29 years old and elected to the Senate," Biden said at the time.
A plain-spoken senator, he is also known for the occasional, costly verbal gaffe. During his latest run for president, he drew fire for describing Obama as "the first mainstream African-American who is articulate and bright and clean and a nice-looking guy." And a speech partially plagiarized from a British Labor leader torpedoed his 1988 run for the White House.
But despite those rhetorical problems, the vice presidential nomination would be another step in a political career that has spanned more than three decades. Elected to the U.S. Senate as a Democratic wunderkind in 1972 at age 29, Biden is currently in his sixth term representing Delaware.
As chairman of the Foreign Relations Committee, he criticized President Bush's handling of the wars in Iraq and Afghanistan. In his speech announcing his presidential run, Biden described the Iraq war as "the greatest foreign policy disaster of our time." He also put forward the controversial idea of partitioning Iraq into three largely sovereign regions -- one for Kurds in the north, Sunni in the center and Shiites in the south.
Prior to the Senate, Biden served on the New Castle County Council in Delaware from 1970 to 1972 and was a practicing attorney in Wilmington.
In his November interview, he reflected on how his personal challenges had affected his outlook on politics.
"[T]here's millions of Americans who have gone through difficult things. So I don't know how it affects the voters, but I know how it affected me," Biden told Judy Woodruff. "It makes me realize that there's nothing that is critical other than life and death and that there is a solution to almost every problem."