DES MOINES, Iowa — Democrats are preparing for what could be their most contentious debate yet as the leading candidates gather in Iowa on Tuesday looking for a way to break out of the crowded top tier less than three weeks before the state’s caucuses kick-start the presidential nomination process.

Some of the fiercest clashes could center on Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts, fellow progressives who until now have largely avoided criticizing each other.

But Warren chastised Sanders over the weekend following a report that his campaign instructed volunteers to speak poorly of her to win over undecided voters. The tensions escalated on Monday after CNN reported Sanders told Warren in a private 2018 meeting that he didn’t think a woman could win the election, a charge that Sanders vigorously denied but that Warren confirmed later Monday.

The feuding will likely 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 to highlight her candidacy as she remains mired in the middle of the pack in polling. Billionaire Tom Steyer will have to answer criticism that he’s buying 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 some momentum before the Feb. 3 caucuses.

Those shifting dynamics mean Tuesday’s debate could be unlike any of the others that came before it this cycle. The generally polite disputes over policy items including health care and immigration are poised to be replaced by increasingly bitter and personal knocks. And it will happen as many Democratic voters are just beginning to tune into the race.

“The debates are always important — but this one’s probably the most important for these candidates,” said Scott Brennan, a former Iowa Democratic Party chair and current committeeman. “We’ve got at least four people who are bunched right there together at the top. So how do you break out?”

The debate, which is being held on the Des Moines campus of Drake University and will be 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 will be the first debate since President Donald Trump authorized the killing of Iranian Gen. Qassem Soleimani, which heightened tensions throughout the Middle East.

Biden advisers see the 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.

While Biden acknowledged 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.

John Kerry, the 2004 Democratic nominee and a Biden supporter, previewed Biden’s argument Monday in Knoxville, Iowa. Kerry didn’t mention Iraq or Biden’s evolving positions on U.S. engagement there. The former secretary of state instead kept the focus on Trump and his changing stories about his decision to order Soleimani’s killing.

“The United States of America should never be taken to the brink of war on the basis of changing stories and lies,” Kerry said. “I trust Joe Biden to tell the truth.”

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 that make up much of Biden’s base.

But Sanders is less likely to continue the feud that erupted with Warren over the weekend.

Following a report in Politico that the Sanders campaign had instructed some volunteers to characterize Warren as a candidate for wealthy and well-educated voters in conversations with undecided voters, Warren issued a rare critique of her opponent, saying she was “disappointed” he was instructing staffers to “trash” her and emphasizing the need to nominate a unifying candidate to defeat Trump.

That echoed a new argument the Warren campaign unveiled this weekend: that she is the candidate who can best unify the different factions of the party, a case new endorser Julián Castro made when introducing the senator on the stump in Iowa.

While Sanders and Warren have, until this weekend, publicly defended each other on the debate stage and in media interviews, Sanders’ team says they’re 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 their 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.”

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 on the stage and avoided taking the moderators’ bait in going 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 with a standout debate moment.