As six Democratic presidential candidates take the debate stage here in Iowa on Tuesday night, less than three weeks before the vital first caucuses, tensions among the contenders have risen to a new level.

The non-aggression pact between Senators Elizabeth Warren and Bernie Sanders took a hit after news broke that the Sanders campaign had been urging volunteers to describe Warren as the preferred candidate of wealthy voters and then appeared to collapse completely in a row over Sanders’ alleged remarks about the viability of a female candidate.

Meanwhile, the former vice-president Joe Biden, the former South Bend, Indiana, mayor Pete Buttigieg, and Senator Amy Klobuchar – the more moderate candidates within the Democratic presidential primary – are all making arguments on electability and their fitness to rally voters in moderate and conservative parts of the country.

The final Democratic presidential debate ahead of the caucuses is shaping up to be a full-throated brawl highlighting the underlying divides within the Democratic presidential primary.

It has also caused a level of uncertainty about how stable any of the Democratic frontrunners’ positions in the primary really are amid widespread party fears that it is fumbling its choice of opponent to face Donald Trump, who will use incumbency and a strong economy to fight for re-election.

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“Here’s the problem in Iowa right now: any of the top four could finish in any of the top four,” said Steve Elmendorf, a veteran Democratic strategist. “And there’s a big difference between first and fourth. So if you’re Joe Biden, do you want to go after Bernie, do you want to go after Elizabeth? I mean there’s a lot of permutations here – a lot of strategic decisions you’ve got to make.”

Recent polling of the race has been static, with Biden leading the field by just a few points, enough for his rivals to make the case that despite being a frontrunner, the former vice-president is hardly a lock at this point in the Democratic primary.

Most polls have shown Biden followed closely by a regular set of three or four of his competitors: Sanders, Buttigieg, Warren, and Klobuchar trailing the rest. A recent Monmouth University poll found Biden leading the primary field in Iowa by six percentage points. Similarly, a national Quinnipiac University poll of Democratic voters found Biden leading the rest of the field by single digits, with Sanders close behind, ahead of Warren and Buttigieg.

Those poll results have underscored the priorities for the various candidates taking the debate stage. Biden is looking to retain his lead in the final weeks before the caucuses and leverage a victory here to quell voter curiosity about other candidates in the primary.

Facebook Twitter Pinterest Supporters of Amy Kobluchar demonstrate at the Drake University campus in Des Moines on Tuesday, before the debate. Photograph: Kerem Yucel/AFP via Getty Images

“The last couple of weeks here makes a gigantic difference,” Biden told staffers in Des Moines, going on to note that the race is essentially a “toss-up” going into the debate.

Meanwhile, after spending most of the campaign refusing to go after each other, Sanders and Warren are finally lobbing attacks at each other. The trigger was the talking points distributed by the Sanders campaign to volunteers who encounter Warren supporters. During a campaign stop over the weekend Warren said she was disappointed that Sanders was trying to “trash” her campaign, an escalation of the lighter contrasts and comments Warren has made about her progressive rival.

On Monday, relations between the two senators deteriorated further when CNN reported that during a private meeting between Warren and Sanders in 2018, the Vermont senator said that he did not think a woman could win the presidential race.

Warren, in an illustration of her new willingness to bash Sanders, confirmed the report’s account.

“Among the topics that came up was what would happen if Democrats nominated a female candidate,” Warren said in a statement about the meeting. “I said that a woman could win; he disagreed.”

Sanders and his team have strongly disagreed with the description of the meeting.

Jim Messina, manager of Barack Obama’s 2012 re-election campaign, warned the sparring between the two senators could hurt both with Iowa voters.

“Warren and Sanders are competing for the same voters,” Messina said in an email. “It’s inevitable as we get closer to actual voting that the campaigns will draw contrast with each other. That said, I’m skeptical that traditional negative campaigning works with these voters. These are ‘Hope’ voters who want a better world. That tends to make negative lines of attack more difficult.”

Looming over the entire Democratic primary, though, is anxiety over electability. Publicly, and privately, in the closing days ahead of the caucuses and the debate, presidential candidates themselves have focused their closing arguments on electability and beating Donald Trump.

“Joe Biden is the candidate who can beat Donald Trump and repair the damage this president has done to our nation and the world,” Biden’s campaign manager, Greg Schultz, wrote in a memo to donors obtained by the Guardian.

Top-tier campaigns have been frantically rolling out endorsements of Iowa Democratic lawmakers and high-profile party officials are traveling around the state to make the electability argument.

Over the weekend, Warren campaigned with the former housing secretary Julián Castro. Buttigieg’s campaign rolled out a coveted endorsement from Dave Loebsack, one of three Democratic members of Congress from Iowa.

Facebook Twitter Pinterest John Kerry greets attendees during a campaign stop to support Biden at the Biden for President Dubuque office in Dubuque, Iowa, on Sunday. Photograph: Jessica Reilly/AP

The former secretary of state John Kerry, who won the Iowa caucuses in the 2004 presidential race, warned during an appearance with local Democrats in Grundy county that Trump wanted Democrats to engage in an extended primary and pointed to the large amount of polling that shows Biden as the most formidable candidate in a head-to-head matchup against Trump.

“Let’s get real here: who’s going to beat Donald Trump? Every poll that you look at today of the nation shows that Joe Biden’s the only person that consistently beats him in every state except for a couple where he’s tied,” Kerry said. “Nobody else does that.”