Sanders denies telling Warren a woman can't win
Presidential candidate Bernie Sanders has denied telling his Democratic rival Elizabeth Warren that a woman could not win the presidency.
Mr Sanders is accused of making the remarks as the two had a private meeting in 2018, as both were preparing bids for the White House.
"Bernie and I met for more than two hours in December 2018 to discuss the 2020 election, our past work together and our shared goals," Ms Warren said in a statement.
"Among the topics that came up was what would happen if Democrats nominated a female candidate. I thought a woman could win. He disagreed."
Mr Sanders, a senator from Vermont, vehemently denied making the comment.
"It's sad that, three weeks before the Iowa caucus and a year after that private conversation, staff who weren't in the room are lying about what happened," Mr Sanders said in a statement.
"Do I believe a woman can win in 2020? Of course! After all, Hillary Clinton beat Donald Trump by three million votes in 2016."
The accusation comes at a very inconvenient time for Mr Sanders.
The pivotal Iowa caucus, the first vote in the presidential nomination process, is just a few weeks away.
Polls show Mr Sanders potentially clinching a victory there and in the subsequent New Hampshire primary.
Ms Warren is within reach of an upset victory.
But a stoush between Mr Sanders and Ms Warren would be welcome news for the two other contenders, former Vice President Joe Biden, and former South Bend Mayor Pete Buttigieg.
As the quartet duke it out, it has left little room for insurgent candidates. Senator Cory Booker, who was considered a potential frontrunner when he announced last year, dropped out of the race overnight.
Campaign turns nasty
The accusation against Sanders comes as the Democratic primary takes a negative tone.
On Saturday it was revealed Sanders volunteers had been given a script for speaking to potential voters on the phone.
The script indicated to volunteers how they should express doubts about the other candidates in a way that seemed genuine.
The script said Ms Warren was a candidate for "the elite", Mr Buttigieg had no black supporters, and "nobody is really excited" about Mr Biden.
"I was disappointed to hear that Bernie is sending out his volunteers to trash me," Ms Warren said.
"We all saw the impact of factionalism in 2016, and we can't have a repeat of that."
The Iowa caucus takes place on February 3.