A new dispute between Elizabeth Warren and Bernie Sanders marks a significant turning point in a US Democratic primary campaign that has generally been characterised by genial differences over policy.
On Monday, Ms Warren said Mr Sanders told her during a private meeting two years ago that he did not think a woman could win the White House.
Mr Sanders has denied telling Ms Warren that, and a senior Sanders adviser says that “those conversations can sometimes get misconstrued”.
The feud brewing between Ms Warren and Mr Sanders will likely change the tone of the campaign going into Tuesday’s debate.
The Democratic primaries will determine the nominee who will challenge President Donald Trump for the keys to the White House in November’s election.
Voting begins in the Democratic Party contest next month.
On Monday Cory Booker became the latest Democratic party hopeful to withdraw from the race.
Mr Trump could not resist a dig at the Democrats, tweeting: “Bernie Sander’s volunteers are trashing Elizabeth “Pocahontas” Warren. Everybody knows her campaign is dead and want her potential voters. Mini Mike B is also trying, but getting tiny crowds which are all leaving fast. Elizabeth is very angry at Bernie. Do I see a feud brewing?”