One of the most recognizable names in U.S. politics, Biden served for two terms as Barack Obama's vice president after nearly four decades as a senator from Delaware. His high-profile, working-class background and connection to the Obama years would help him enter the race as a front-runner, though he faces questions about his age and whether his more moderate record fits with a party that has become more liberal.
With a record in elected office that stretches half a century, Biden faces multiple challenges.
Last month he struggled to respond to a complaint from Lucy Flores, a 2014 lieutenant governor nominee in Nevada, that he made her uncomfortable by touching her shoulders and kissing the back of her head before a campaign event. A few other women have made similar claims, though none has alleged sexual misconduct.