Obama has been a largely silent figure since he left the Oval Office, to the disappointment of many.But on Wednesday night, America's 44th President will praise his former Vice-President, and accuse Donald Trump of undermining US democracy with his assault on voting rights.
It comes as the party has formally nominated Biden as its candidate - an entertaining (if slightly bizarre) television extravaganza that merged presidential politics with the production values of a game-show.
Robert Moore reports on the formal nomination of Joe Biden as the Democratic candidate in the US election
But hidden from view by party managers is the fact that the Obama-Biden relationship is not straightforward.Biden was aggravated that Obama put his weight behind Hillary Clinton four years ago. Obama is known to view Biden as someone who hasn't the focus or the intellectual self-discipline to be president.But in the era of Trump, all is forgiven and the party is swinging decisively behind Biden.
For Democrats, America is facing an existential danger and there is no longer the time to engage in policy arguments or personality clashes.So Biden is emerging from this Convention strengthened.But no Democrat should be complacent.
They felt confident four years ago, and then their coalition of voters failed to turn out in key battleground states.So the next two days are about boosting Biden - painting him as the indefatigable fighter for the little guy, the nation's healer-in-chief, an empathetic figure who can unite America - and about mobilising supporters.