Bernie Sanders, the Vermont senator and second-placed candidate who challenge Hillary Clinton for the nomination in 2016, has also seen his poll numbers dropping.
Elizabeth Warren, the left-leaning senator from Massachusetts, is now within touching distance of overtaking him in early voting states. The eventual winner will take on Donald Trump at the November 2020 election.
Twenty different Democrats will get the chance to make their case during two consecutive debate nights, 10 people in each, in Detroit on Tuesday and Wednesday.
For those struggling to get above 1 per cent in nationwide polls time is running out to make an impact, meaning there is more motivation to create ‘moments’ which can dominate TV coverage of the clashes.
Mr Biden, who served as eight years as Barack Obama’s deputy, will appear in Wednesday’s debate, as will both Ms Harris and Mr Booker.