How unprecedented is Joe Biden's sudden withdrawal from the presidential race? He's given his endorsement to Kamala Harris to take his place, but not all Democrats have, including former president Barack Obama. How will a new candidate be chosen? Dan is joined by Professor of American History at the University of Cambridge Gary Gerstle to look back to 20th-century presidential campaigns that may shed light on what could happen next and unpick the 'political conventions' process of electing a new candidate.

Produced by Dan Snow, Mariana Des Forges & edited by Dougal Patmore

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