DES MOINES, Iowa — New tension between the Democrats' progressive stars threatened to shake up the party's presidential nomination fight Tuesday night in the final debate showdown before primary voting begins.

Just six candidates gathered in Des Moines, each eager to seize a dose of final-days momentum on national television before Iowa's Feb. 3 caucuses. But a sudden “he-said, she-said” dispute over gender involving two longtime allies, Bernie Sanders and Elizabeth Warren, loomed over the event.

Specifically, Warren charged publicly on the eve of the debate that Sanders told her during a private 2018 meeting that he didn't think a woman could defeat Trump, a claim tinged with sexism that Sanders vigorously denied. Amid an immediate uproar on the left, there were signs that both candidates wanted to deescalate the situation, yet they were almost certain to be pressed to confront during the debate.

The feuding was likely to expand to include nearly every candidate on stage. Sanders has recently stepped up his attacks on former Vice President Joe Biden over his past support of the Iraq War, broad free-trade agreements and entitlement reform, among other issues. Minnesota Sen. Amy Klobuchar, who has had several strong debates, will be looking for another opportunity as she remains mired in the middle of the pack in polling. Billionaire Tom Steyer will have to answer criticism that he's trying to buy his way to the White House.

And with two surveys showing Pete Buttigieg losing support in Iowa, the former mayor of South Bend, Indiana, will need a breakout moment to regain strength before the caucuses.

Democratic presidential candidate former South Bend, Ind., Mayor Pete Buttigieg, speaks with a member of the audience at a campaign stop at the Madison County Fairgrounds, Monday, Jan. 13, 2020, in Winterset, Iowa.
Andrew Harnik / AP
Democratic presidential candidate former South Bend, Ind., Mayor Pete Buttigieg, speaks with a member of the audience at a campaign stop at the Madison County Fairgrounds, Monday, Jan. 13, 2020, in Winterset, Iowa.

The shifting dynamics meant Tuesday's debate could be unlike any of the others in this cycle. Generally polite disputes over policy items including health care and immigration are being replaced by increasingly bitter and personal knocks — just as many Democratic voters are beginning to tune into the race.

"We’ve got at least four people who are bunched right there together at the top. So how do you break out?” said Scott Brennan, a former Iowa Democratic Party chair and current committeeman.

Not to be forgotten amid the focus on the Democratic race, President Donald Trump’s reelection campaign flew a banner in the skies over Des Moines for several hours before the debate. Trump himself countered by hosting a campaign rally in Milwaukee.

The debate, on the campus of Drake University and televised on CNN, marks the first forum with an all-white lineup. Businessman Andrew Yang, an Asian American candidate who appeared in the December debate, failed to hit the polling threshold for Tuesday's event. And New Jersey Sen. Cory Booker ended his campaign on Monday after he didn't make the debate stage, leaving just one black candidate — former Massachusetts Gov. Deval Patrick — in the race.

This was the first debate since Trump authorized the killing of Iranian Gen. Qassem Soleimani, which heightened tensions throughout the Middle East.

Biden advisers see that development as a boon to his candidacy, allowing him to argue he’s a steady, experienced alternative to Trump. But it could easily become a problem if Biden fails to answer what will likely be pointed attacks from Sanders on his support for the Iraq War.

Former Vice President and Democratic presidential candidate Joe Biden speaks at a campaign event Saturday, Jan. 11, 2020, in Las Vegas.
John Locher / AP
Former Vice President and Democratic presidential candidate Joe Biden speaks at a campaign event Saturday, Jan. 11, 2020, in Las Vegas.

While Biden said over a decade ago his vote was a mistake, he’s struggled to offer a clear answer for his support, at times misleadingly asserting that he opposed the war from the start.

Sanders is eager to take the fight to Biden, as his advisers believe his message on income inequality and major structural change can appeal to the same white working-class voters who make up much of Biden's base.

While Sanders and Warren have, until this weekend, publicly defended each other on the debate stage and in media interviews, Sanders’ team was expecting attacks from the Massachusetts senator on the debate stage. They believe that the Warren campaign is responsible for leaking what they say is an inaccurate description of the senators’ 2018 private meeting, as reported by CNN. But in a statement later Monday, Warren said the description of their meeting was correct.

“Among the topics that came up was what would happen if Democrats nominated a female candidate. I thought a woman could win; he disagreed,” Warren said in a statement. “I have no interest in discussing this private meeting any further because Bernie and I have far more in common than our differences on punditry.”

Democratic presidential candidate Sen. Elizabeth Warren, D-Mass., left, poses for a photo with attendees after speaking at a campaign event, Sunday, Jan. 12, 2020, in Marshalltown, Iowa.
Patrick Semansky / AP
Democratic presidential candidate Sen. Elizabeth Warren, D-Mass., left, poses for a photo with attendees after speaking at a campaign event, Sunday, Jan. 12, 2020, in Marshalltown, Iowa.

Trump, campaigning in neighboring Wisconsin, weighed in on the feud.

“She said that Bernie stated strongly that a woman can’t win. I don’t believe that Bernie said that, I really don’t. It’s not the kind of thing Bernie would say," Trump said.

The clashes could offer an opportunity for candidates who stay above the fray. While Klobuchar sparred with Buttigieg during the last debate, she’s previously sought to tamp down tensions among her opponents and avoided taking moderators’ bait to go after other candidates.

And Steyer, who has largely flown under the radar throughout the campaign, could look to capitalize on a handful of recent polls that have shown him gaining traction in some of the early primary states.

Peoples reported from Washington. Associated Press writers Bill Barrow in Des Moines and Darlene Superville in Washington contributed to this report.