Winterset, Iowa (CNN) Six of the Democratic presidential candidates meet for the final debate before the Iowa caucuses on Tuesday at a strikingly fluid moment in the 2020 race when it appears any one of the top four contenders could emerge as the victor in the Hawkeye state.

In tonight's CNN/Des Moines Register debate , the Democratic hopefuls are all trying to solidify their support among the state's many undecided voters, trying to lock down Iowans who will caucus for them on February.

A recent flurry of polls suggests the top four hopefuls are locked in something close to a four-way standoff, with former Vice President Joe Biden , former South Bend, Indiana, Mayor Pete Buttigieg , Vermont Sen. Bernie Sanders and Massachusetts Sen. Elizabeth Warren all clustered at the top of the field.

More than a dozen interviews with likely caucus goers over the past two days underscored that indecision with many telling CNN they are still deciding among three to four candidates, including some who did not make the cut for the debate stage. Tuesday's matchup at Drake University also comes at a time when the candidates are beginning to jab their rivals more forcefully -- injecting a new air of unpredictability into the contest.

Iowans are famous for making up their minds at the last moment -- even as late as the cold night when they gather to caucus.

"This is the weirdest caucus season ever," said Lisa Ann Spilman, a 52-year-old veteran and undecided voter who said she has been a caucus precinct captain many times. "There are so many great candidates. We keep having our one, two, three -- then somebody drops out or somebody needs help and then you re-think it," she said, noting that she has canvassed this cycle for Montana Gov. Steve Bullock, Colorado Sen. Michael Bennet, Warren and Biden -- in part to help make up her own mind.

On Monday, she spoke up as a victim of sexual assault in the military during Buttigieg's town hall in Winterset, asking the former South Bend, Indiana mayor to commit to signing legislation known as the Military Justice Improvement Act that aims to address and prevent sexual assault in the military. Buttigieg said he would sign the bill, but Spilman she was still undecided after his event and wanted to talk to the other candidates about the legislation.

"We in Iowa are so lucky that we get to see people two, three, four times," she said, noting that gives them all "a chance."

Campaign gets more heated

In recent days, the Sanders campaign has moved into a more aggressive posture, using the US-Iran tensions as an opportunity to highlight what they see as Biden's poor judgment in voting to invade Iraq. This past weekend also seemed to mark the end of Sanders' unspoken non-engagement pact with Warren, a development that seemed inevitable given the ideological closeness of the two and the fact that they are currently splitting the progressive wing of the Democratic Party.

Warren seized on a report from Politico on Saturday that Sanders volunteers were using a call script aimed at convincing voters that she will only be able to attract "highly educated, more affluent people" to her campaign -- making her a less viable nominee against President Donald Trump.

Though Sanders disavowed the talking points memo -- stating he'd just heard about and pointing out that he never criticizes Warren on the campaign trail -- her campaign immediately began fundraising off the document. Warren told reporters she was disappointed Sanders "is sending his volunteers out to trash me."

Sanders was once again on the defensive Monday after Warren said in a statement that Sanders told her in a private 2018 meeting that a woman could not win the presidency.

"Among the topics that came up was what would happen if Democrats nominated a female candidate," Warren said.

"I thought a woman could win; he disagreed."

Her revelation backed up the account four sources had shared with CNN in an earlier report.

In response to that report, Sanders insisted he said no such thing, calling the claim "ludicrous."

"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," Sanders told CNN.

The sudden eruption of negative campaigning in the midst of what had been a relatively congenial race seemed all but certain to change the dynamics of an already volatile race.

A changing race

While the two most progressive candidates were off the trail Monday, the other top two -- Buttigieg and Biden -- campaigned in Iowa, each vying to become the top choice of voters who want a more moderate, middle-of-the-road candidate.

Buttigieg is looking to regain the momentum he seemed to have in Iowa before the holidays. A new CNN/Des Moines Register/Mediacom poll released Friday showed the 37-year-old mayor sliding back as Sanders gained ground among likely caucus goers at 20%.

Sanders also gained ground in a new Monmouth University poll released Monday, notching the support of 18% of likely Iowa caucus goers. In that survey, Biden took the top spot at 24%. But only four in 10 of likely caucus goers told Monmouth they were firmly decided.

The CNN/Des Moines Register debate at Drake University comes as the field of candidates has narrowed to a far less diverse group than last year. Sanders, Warren, Biden and Buttigieg will be joined on the debate stage by Minnesota Sen. Amy Klobuchar and Businessman Tom Steyer.

New Jersey Sen. Cory Booker dropped out of the race on Monday after failing to qualify for the debate. Businessman Andrew Yang , who lamented the shrinking diversity of the field at the last debate, also could not meet the polling and fundraising thresholds set by the Democratic National Committee.

Steyer managed to make the cut after the release late last week of two new Fox News polls, which found him at 12% among likely Democratic caucusgoers in Nevada and one where he was at 15% among South Carolina likely Democratic primary voters.

Iowans still making up their minds

Iowa voters continued on Monday to put the candidates through their paces.

At Buttigieg's Winterset town hall Monday, 27-year-old Albin Mehmedovic aggressively questioned Buttigieg's commitment to working class voters given that he holds high-dollar fundraisers that average people can't afford to attend.

"Don't you think that the working class Americans like teachers deserve an explanation of why you're not as transparent about these types of meetings," Mehmedovic asked Buttigieg in Winterset where he works as a roof inspector. "How do we know that you're not making deals behind the American people's backs with these corporate interests in mind?"

Buttigieg pointed out that he now opens his fundraisers to reporters (a decision he made after facing heat from Warren late last year).

"We're going up against a President who's going to pull out the all of the stops in order to stay in power, they've already put together 300 million bucks and they're not going to stop," the former mayor said. "And we need to go into this fight with everything that we've got."

After the town hall, Mehmedovic said he wasn't satisfied with Buttigieg's answer and would likely caucus for Sanders.

"I don't feel as connected to a candidate who's willing to hold these private meetings. ... A lot of that stuff is not revealed to the public eye," Mehmedovic said in an interview, describing the practice as "dodgy" and pointing out that Sanders and Warren are "grassroots funded."

"I really like Bernie because I feel like he's genuine. He's kind of kept the same thing consistent throughout time," Mehmedovic said. "With somebody like Bernie I can fully get behind him."

Another voter at Buttigieg's event, 76-year-old Maxine Bussanmas of Bevington, said she was still intrigued by Buttigieg's intelligence and charisma but that she has decided to caucus for Biden, because she thinks Buttigieg needs more seasoning before he'll be ready for the White House.

"Right now we need experience and we also need international good will," said Bussanmas. "World leaders respect him," she said of Biden. "Our standing in the world has been totally tanked by our President. Something new crops up every day."

On the other side of the experience versus judgment debate that Biden and Buttigieg have been engaged in on the trail, Winterset voter Connie Bosier said she decided to caucus for Buttigieg.

"He seems to care about everyone, not just one group or another," said Bosier, who is 69 and works with people with disabilities. "I'm really not a Biden fan, because I really think that he's already had the chance," she said, adding she believes Sanders' time has also passed. "And Elizabeth Warren just kind of seems like an up-and-down person -- you know, (you) listen to her and then you're kind of like, 'Ah, I'm not sure.' So I think that's why Pete is more of the kind of person I like."

But many voters were still in the predicament of Mike and Vicky Brenner: undecided. The couple hosted Jill Biden at their house Sunday night and Vicky Brenner introduced Buttigieg on Monday in Winterset. Mike Brenner said right now he supports Biden, Buttigieg and Bennet "even-steven."

Vicky Brenner said she's still neutral: "I love all of our candidates. It's a great problem to have."

"I keep telling these staffers don't get nervous. And if people haven't committed, do not try to strong arm them into, they'll make their decision," Vicky Brenner said. "We're Iowans, and we're independent thinkers and we don't like people kind of pushing us."

Update: This story has been updated with more of Warren's statement Monday night.