"Barack has chosen Senator Joe Biden to be our VP nominee," the text message, sent at around 3 a.m. ET, said.
"Joe and I will appear for the first time as running mates this afternoon in Springfield, Illinois --- the same place this campaign began more than 19 months ago," Obama said in an e-mail sent to supporters Saturday morning.
"I'm excited about hitting the campaign trail with Joe, but the two of us can't do this alone," he wrote. " We need your help to keep building this movement for change."
Vice presidential candidates are often called on to be the aggressors in the campaign. A comment Biden made earlier this year may have resonated with Obama: