Article published on the 2008-08-23 Latest update 2008-08-23 11:34 TU
The long-awaited announcement came as most Americans were asleep but, although Obama had promised to tell his supporters first, it had already been leaked to the news-media.
"I've chosen Joe Biden to be my running mate," Obama said in an email and text message to supporters. "I'm excited about hitting the campaign trail with Joe, but the two of us can't do this alone. We need your help to keep building this movement for change."
US television channels had already predicted the choice thanks to leaks and the fact that a secret service detail was reported to have been sent to protect Biden.
Obama had earlier told two other leading contenders, Indiana Senator Evan Bayh and Virginia Governor Tim Kaine, while his chief rival for the nomination, Hillary Clinton, had already been ruled out.
Biden supported the Iraq war at its start but has since become one of its most vocal critics.
Obama's camp hopes the choice will bring political experience and an air of authority on foreign policy questions to the campaign but Biden also has a reputation for verbal gaffes.
In a bid to divide Democratic supporters, the McCain camp cited criticisms of Obama made by Biden during his own bid for the party's presidential nomination.
"There has been no harsher critic of Barack Obama's lack of experience than Joe Biden," McCain spokesman Ben Porritt said in a statement.
"Biden has 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," Porritt said.