Elizabeth Warren and Bernie Sanders have disagreed in a Democratic presidential debate over whether he once told her a woman could not win the White House in 2020.
The disagreement underlines an emerging rift between the progressive allies as the first voting nears.
After days of tensions between the fellow US senators, friends and liberal standard-bearers, who agreed early in the campaign not to attack each other, Sanders emphatically denied he ever made the remark, saying it was "incomprehensible" he could have said such a thing in a private 2018 meeting with her.
Warren confirmed the comment and said she disagreed with Sanders, but quickly pivoted to the broader question of whether a woman could be elected president.
"Bernie is my friend and I am not here to try to fight with Bernie. But look, this question about whether or not a woman can be president has been raised and it's time for us to attack it head-on," Warren said in the debate in Des Moines, Iowa.
The dispute brought questions about gender, sexism and electability back into the spotlight in the campaign, almost four years after Democrat Hillary Clinton failed in her bid to become the first woman president, in an upset loss to Republican Donald Trump.
Warren pointed out the men on the stage had collectively lost 10 elections, while the two women on the stage - she and US Senator Amy Klobuchar - had won each election they had contested.
Klobuchar pointed to the 2018 election of women governors in conservative states as further evidence a woman can win the White House.
The rare clash between Warren and Sanders came as recent opinion polls show her trailing him nationally and in early nominating states just weeks before Iowa voters kick off the Democratic nominating race on February 3.
The debate also featured sharp exchanges between Sanders and former Vice President Joe Biden on foreign policy and trade as the top Democrats made their case to voters assessing an unsettled presidential field with no clear front-runner.
The debate was the seventh in the race to find a November election challenger to Trump. Polls show an extremely tight race among Biden, Sanders, Warren and former South Bend, Indiana, Mayor Pete Buttigieg.
Sanders, a longtime antiwar advocate who voted against the 2002 authorisation of war in Iraq, criticised Biden for supporting the war and said they heard the same arguments from officials in former President George W. Bush's administration before coming to different conclusions.
"I thought they were lying, I did not believe them for a moment," Sanders said. "I did everything I could to prevent that war. Joe saw things differently."
Biden, a former chairman of the Senate Foreign Relations Committee who touts his security credentials, acknowledged the vote "was a big, big mistake" and said that as President Barack Obama's vice president, he worked to bring the troops home.
"It was a mistake to trust that they weren't going to go to war," Biden said of the Bush administration. "It was a mistaken vote, but I think my record overall on everything we have done, I'm prepared to compare it to anybody's on this stage."
With surveys showing a virtual tie in Iowa and a largely undecided electorate, all the candidates face mounting pressure to make an impression.
Australian Associated Press