DES MOINES — The Democratic presidential candidates returned to Iowa in full force Saturday, using a brief break from impeachment proceedings to rally supporters ahead of Monday's caucuses with renewed pitches to an electorate that remains highly skittish and deeply undecided.

Joe Biden used his closing argument to present himself as the safest choice for voters worried most about finding a nominee who can defeat President Trump. Sen. Elizabeth Warren (D-Mass.), who like other senators in the race had been stuck in Washington until this weekend for the impeachment trial, made an explicit appeal to women — and pitched herself as the one who can, as signs behind her read, “Unite the Party.”

Pete Buttigieg’s campaign tried to rally the party behind his call for generational change, while Sen. Bernie Sanders (I-Vt.), using weekend concerts and rallies, sought to mobilize what many strategists here consider to be an army of dedicated supporters.

Several of the candidates played to overflow crowds, a contrast to earlier in the week when the absence of the senators damped the energy normally associated with the final days.

Virtually every campaign has come to predict privately that Sanders could turn out more people on Monday night than any other candidate. Due to the arcane rules of the Iowa caucus system, his rivals hope they can overcome that advantage when the final delegate counts are tallied.

“I think it’s going to be a cluster,” Biden said in an interview after an event late in the week. “It’ll be relatively close, you know, probably three of us that are fairly close.”

Biden and Sanders have been at the top of most polls in Iowa this month. But Democrats hoping for more clarity got a unwelcome surprise Saturday night when the Des Moines Register, CNN and Selzer & Co. pulled back from the much-anticipated release of the results from the final Iowa poll, long considered the most reliable pre-caucus snapshot.

The decision, which added to the chaos and uncertainty in the final days, came after the Buttigieg campaign alerted those overseeing the poll that his name was left off of the list of candidates read by one of the interviewers, according to a person familiar with the problem who spoke on the condition of anonymity.

With Sanders appearing to surge in recent weeks, all of his rivals were eagerly working to manage expectations. Some close to Warren said privately that the fight was now for a second-place finish. Those in Buttigieg’s camp say they believe finishing ahead of Biden would provide the springboard they need for future contests, both to build support and to reload their campaign war chest.

“It’s so tight that we just got to keep our heads down and maintain a kind of an underdog mentality all the way through,” the former mayor of South Bend, Ind., said in an interview. When pressed about what expectations he needed to exceed, he demurred, but did say, “We’re in it to win it.”

The hunger to defeat Trump — and the likelihood the Senate will vote to acquit him this week — has had the effect of leaving many voters immobilized, creating an unusually large undecided pool and injecting more than the usual uncertainty over how the coming days will play out.

Among the overriding questions still hovering at the start of what could be a long fight for the nomination is whether the continued indecision will dampen participation Monday. Earlier predictions for record turnout have been scaled back in the past weeks.

The caucuses Monday night will be the culmination of more than a year of campaigning and a steady churn of candidates entering and exiting. Collectively, the campaigns have spent $70 million on TV ads here. When they are done, Iowans will have played their traditional role of stress-testing — and winnowing — what had been a historically large and diverse field.

While the party over the past three years has struggled to answer a basic question — How do you beat Trump? — the results here will offer the first moment of clarity. It is a test of whether the party wants to move toward candidates preaching bold and unsettled change that would bring a Democratic revolution to counter Trump’s Republican one, or whether the party wants a more traditional, return-to-normalcy nominee who would run on more kitchen-table, uncontroversial issues.

In what is the starting gun for a sprint toward contests that will grow more and more costly — and with Mike Bloomberg pouring hundreds of millions of his own money into the race — several campaigns are in dire need of a win that could fuel donations into campaign accounts that have dwindled.

The contest has been largely free of the kind of negative campaigning that has marked other years, but that hasn’t eliminated sniping. Top advisers to Buttigieg on Saturday morning went after Biden, suggesting that his long career in Washington would be a detriment to the party’s ability to defeat Trump if the former vice president is the nominee.

“The idea that we are going to take on someone like Donald Trump with the old playbook by saying I understand the ways of Washington, I hung out with Strom Thurmond, you know, 20 years ago — that’s not going to happen,” Lis Smith, a senior Buttigieg adviser, told reporters during a breakfast hosted by Bloomberg News.

Mike Schmuhl, the campaign manager, later jumped in with another data point.

“Since World War II, our party has nominated three vice presidents. They’ve all lost,” he said. “Our party goes for youthful, visionary, next-generation leaders.”

Warren has had a prized organization in the state — along with several late, sought-after endorsements — but has also fallen in recent polls as others have cut into different parts of her base. She is competing with Buttigieg for college-educated voters, with Sanders for liberal voters and with Sen. Amy Klobuchar (D-Minn.) for voters eager to see a woman as the nominee.

Ahead of Warren’s event Saturday in Iowa City, a group of elected officials tried to lead the crowd in a chant: “It’s time! It’s time! It’s time for a woman in the White House!”

Warren has generally avoided questions about how she might do in Iowa. But her campaign manager Roger Lau recently sent out a memo warning of the “breathless media narratives” likely to emerge from the early states and focusing on their 1,000-strong staff fanned out in 31 states.

Klobuchar has cast herself as a not-too-hot, not-too-cold candidate who can appeal to a wide swath of the party. But what is unknown is whether the fact that she, along with Warren and Sanders, had to be in Washington during the final week of campaigning had hindered her ability to capi­tal­ize on the fresh look many were giving her.

Entrepreneur Andrew Yang and businessman Tom Steyer have each averaged around 4 percent in the polls, which would put them below the 15 percent viability threshold. But their supporters could play a significant role in determining the night’s final outcome if they switched to another candidate on the second tally.

With concern bubbling among establishment Democrats over whether Iowa could launch Sanders forward with momentum, the party was caught up Saturday in a replay of 2016 tensions between Sanders and Hillary Clinton.

After Clinton again criticized Sanders, Rep. Rashida Tlaib (D-Mich.), a top Sanders surrogate, playfully urged a crowd to boo Clinton during a rally on Friday night. But in yet another sign of the skittishness of most campaigns to get into a bitter brawl, Tlaib apologized Saturday and Sanders emphasized that he would support whoever the nominee is.

Sanders’s campaign advisers say they have identified as many supporters who are coming to caucus Monday, in a crowded field with at least five competitive candidates, as came to caucus for him in 2016, when it was essentially a two-person race. They are banking on a large turnout among young voters, and those who typically don’t participate in local politics.

“They have not engaged with the party structure,” said Kurt Meyer, chairman of the Democratic Party in three rural counties. “Maybe that is a great leap forward. Maybe they know something the rest of us don’t and they’ll get an additional 10 or 20 or 30 percent who have never caucused before to magically show up. But I honestly don’t know who my Sanders organizer is.”

Warren grew a bit nostalgic as she returned to Iowa, reminiscing about all of her previous trips. “You’ve whispered dreams into my ears, you’ve told me about your lives, about issues, about ideas, about how we could make things better,” she said in Cedar Rapids. “In this year you have made me a better candidate, and you will make me a better president. Thank you.”

Unlike past campaign stops, Warren did not stay around for a photo line, instead leaving her dog Bailey behind to appear in pictures with supporters.

The caucuses are the first major test of whether Biden’s claims of electability will draw voters to his candidacy. While his events can feel lethargic and with crowds smaller than for other candidates, his campaign advisers have been banking on the goodwill he has built up over nearly five decades in public life.

Asked in an interview about the seeming lack of enthusiasm at his events and whether that worries him, Biden said: “I don’t get a sense of that at all. . . . We spent a lot of time in a lot of small venues, deliberately going to make sure we cover rural Iowa.”

Former Iowa governor Tom Vilsack said Biden is a known commodity and that Iowans who support him don’t feel the need to come to Biden’s rallies but will be there on caucus night. It is a mistake, he said, to assume that “there’s a direct parallel between enthusiastic rallies and turnout and results.”

His campaign organization has also been a frequent topic among Iowa operatives and county chairs, who say it has lacked the kind of vigor needed in a caucus system that rewards passion and organization more than it does widespread but thinner support.

The coalition Biden is trying to assemble includes voters over age 50, blue-collar households, veterans, African Americans, Latinos and Catholics. His campaign is running specific programs focusing each group, based on lists developed from voter files, property tax rolls and other sources.

Because many people no longer answer their telephones unless they recognize the caller’s number, the campaign has put extra emphasis on sending volunteers and paid canvassers to people’s homes.

The decision to hire paid canvassers — offering $20 per hour for the last two weeks — raised eyebrows among some of Biden’s rivals, who took it as a sign of weakness in the organization. But campaign officials say they believe the additional help has allowed them to get into as many neighborhoods as possible and during days of inclement weather.

Although Monday night’s weather is expected to be brisk with no snow in the forecast, it remains to be seen if turnout will be historically high.

While some still predict it could match or exceed the 240,000 who turned out in 2008, others say it is likely to fall between that number and the 171,000 who attended in 2016.

Just who would benefit from a low turnout is a matter of some debate.

Biden’s candidacy tends to attract traditional caucus-goers who might have the added incentive to participate this year because of their desire to defeat Trump in November.

But a lower turnout could give the advantage to candidates who have attracted newcomers, such as Sanders, or who are generating real enthusiasm at rallies.

The campaigns have been preparing for what could be a confusing night of potentially conflicting results, and multiple candidates seeking to shape a public impression that they scored a symbolic victory if not a numerical one.

The state party will report three numbers on caucus night. The first will be the number of people who show up for each candidate. The next two will reflect results after supporters of candidates who do not meet the 15 percent threshold in a precinct realign to support another candidate.

There will be two measures of that realignment, first the raw number of people in support of the remaining viable candidates and then how that translates into delegates — or, as they will be called Monday, state delegate equivalents. Those two numbers should track relatively closely but there could be a notable difference between the entrance percentages and the delegate percentages.

For several candidates — particularly Buttigieg, Klobuchar and Warren — a bad finish could be devastating. It could also begin to clarify the question of whether the campaign is headed toward a long, divisive primary or whether it could conclude more swiftly. The candidates will quickly head to New Hampshire, where there will be an eight-day sprint that includes a Friday debate.

Biden, amid growing concerns that he is having more difficulty raising money, has bought or reserved only $215,000 in New Hampshire, according to Advertising Analytics, which puts him at a fraction of his rivals. His allies are hoping a strong showing in Iowa could trigger a rush of donations, but even then there is some debate over how heavily he will focus on a state that Sanders carried handily four years ago and that neighbors Warren’s home state of Massachusetts.

There have been private deliberations among those leading Biden’s super PAC over how heavily to concentrate on New Hampshire and whether to instead pour resources into Nevada, the next state on the calendar.

But any decision is dependent on what happens in Iowa on Monday night.

Annie Linskey contributed to this report.