DES MOINES — The Democratic presidential field narrowed and the intensity of its survivors heightened this weekend, with one once-major candidate dropping out and a dozen others frantically trying to avoid that fate in a state that has taken on a striking importance in the 2020 race.

As they have nationally, four candidates have emerged in Iowa from the sprawling pool — the largest and most diverse in history — as the leaders in statewide polling: former vice president Joe Biden, Sens. Elizabeth Warren (D-Mass.) and Bernie Sanders (I-Vt.), and South Bend, Ind., Mayor Pete Buttigieg.

Their challengers received a blunt reminder Friday of the stakes three months before the state’s caucuses, when the first votes of the 2020 nomination will be cast, as former Rep. Beto O’Rourke of Texas abruptly left the race. That came days after another trailing contestant, Sen. Kamala D. Harris (D-Calif.), was forced to publicly acknowledge layoffs and consolidations because of her dwindling campaign fundraising.

A series of events in Iowa this weekend marked the beginning of the unofficial sprint to the Feb. 3 caucuses and summoned questions about which candidates would survive until then. All were operating on the hope that Iowa would provide them the sort of political magic that propelled Barack Obama to an upset victory in the 2008 caucuses.

For the trailing candidates, a fundraising dinner for the state party in Des Moines on Friday night was an opportunity to try to break into the top tier — or even just get a little buzz on social media. The evening often felt more like a high school field day competition, but with a blown-up budget, than a sober part of the nomination process.

By the end of the marathon speechmaking, however, it was not clear whether any of the candidates significantly benefited from the exercise or if they just confirmed their status.

Harris, who is shuttering all but one of her campaign offices in New Hampshire to focus more heavily on Iowa, showed up Friday night with a marching band and bought hundreds of tickets for supporters. Sen. Cory Booker (D-N.J.) gave a deeply personal speech about his mentor — and was accompanied by his girlfriend, actress Rosario Dawson. Businessman Andrew Yang held a predinner “Yangapalooza” featuring a member of Weezer. Former housing secretary Julián Castro ignored his time limit. Sen. Amy Klobuchar (D-Minn.), who has recently seen gains in the state, passionately shouted again and again that she could “win big” as a supporter in the audience blew a Gjallarhorn.

The major difficulty for all of them is the uncertainty that has seized the Iowa electorate, which is largely torn as it tries to ascertain the best candidate to go up against President Trump. Even in the far reaches of rural Iowa, Democrats openly fret about electability as if they were television pundits, wondering how a particular candidate will play with voters in states like Wisconsin and Pennsylvania — or how they will fare on the general election debate stage.

“You saw what happened last time,” said Nina Smith, 80, a retiree who attended a recent event for Booker in West Des Moines but remains undecided in the race. “You can love someone so much in your heart, but in your head, you wonder if they can really beat Trump.”

Margaret Torrie, a 72-year-old retiree from Ames, said the pressure of making a decision was so stressful that she was having nightmares.

“The number one issue for me is that I don’t want to see Trump reelected,” said Torrie, an undecided voter who recently attended a Harris event. “But I have these dreams, these almost Technicolor dreams where we’ve picked the wrong person, and I wake up in the middle of the night in a panic.”

The state has been swarming with candidates for months, but it was even more saturated this weekend with the Liberty and Justice dinner and its accompanying festivities Friday, followed Saturday by several issue-focused town halls, campaign rallies and a fish fry in eastern Iowa organized by Rep. Abby Finkenauer, a young Democrat who beat a Republican incumbent in 2018 in a district packed with counties that voted for both Obama and Trump.

The historic number of candidates has exhausted some Iowans, who have grown weary of the relentless contact from numerous campaigns desperate to cultivate every last bit of support. Volunteers from the Warren and Sanders campaigns have accidentally phoned each other during competing phone banking, and some voters say they have stopped answering calls from phone numbers they don’t recognize.

“It’s like 10 people trying to walk through the same door at the exact same time,” said Grant Woodard, a Des Moines attorney and former Democratic operative who is unaligned in the race.

The weekend demonstrated a markedly sharper tone on the part of several candidates. Warren said that Biden was “running in the wrong presidential primary” and warned, without naming names, that nominating a candidate promising “business as usual” after Trump would result in a Democratic loss. Representatives for Sanders and Biden swapped cutting insults in debating the cost of Sanders’ Medicare-for-all proposal. On Friday night, Buttigieg promised to not “get so wrapped up in the fighting that we start to think fighting is the point,” to which Warren countered: “Anyone who comes on this stage and tells you they can make change without a fight is not going to win that fight.”

The Iowa strategies of these candidates mirror how they hope to appeal nationally.

Biden’s approach is similar to those of Democrats in years past, based on an assumption that next year’s caucus-goers will be the usual crowd — mostly white, mostly older. His Iowa staff of about 100 has hyperfocused on turning out senior citizens and working-class Democrats in rural and conservative parts of the state, especially those who voted for Trump. Sanders, who relies heavily on a network of volunteers in addition to an Iowa staff of 112, has been trying to energize his longtime supporters, who include many young voters, and make inroads with Iowa Latinos. Next week, Rep. Alexandria Ocasio-Cortez plans to join him at a series of Iowa rallies.

Buttigieg, who waited until late summer to build his Iowa operation, is making nearly weekly visits to Iowa and like Biden has focused heavily on the counties that voted twice for Obama and then picked Trump in 2016, hoping his more moderate positions and message of unity will resonate. Klobuchar has taken a similar approach and has visited a number of counties that voted heavily for Trump in 2016, touting her ability to win in rural Minnesota counties.

Harris spent 15 days in the state in October and is trying to save money for a seven-digit television ad buy in Iowa in the weeks before the caucuses, according to her staff. Unlike Biden’s staff, hers is focused on voters they hope to pull into the caucus mix, especially young people and “Blue Wave Women” who became politically active after Trump was elected and played a key role in Democratic wins in 2018.

Warren, who is widely credited with having built the most formidable ground operation in the race months ago, has focused on an array of voters, opening campaign offices in both liberal hubs and conservative enclaves.

“Elizabeth Warren is in a league of her own, in terms of the organization, the operation,” said Sean Bagniewski, chairman of the Polk County Democrats, which encompasses the Des Moines metro area. “To me the big story is, who can even put up a fight with her right now?”

She, and to a lesser extent Buttigieg, have risen in state polls as Biden has appeared to weaken. The former vice president has suffered from poor fundraising compared with the other leaders, and last week a super PAC intended to help him make up ground was officially opened by supporters.

Tom Vilsack, the former governor of Iowa and agriculture secretary in the Obama administration, who has not endorsed a candidate, raised questions about Warren’s expansive agenda and how she would push it through a Congress less liberal than she is. He hesitated to evaluate the field’s strengths and staying power.

“I don’t know whether any of these candidates are able to take a punch,” he said.

Warren, Buttigieg and Sanders have attracted large crowds at events throughout the state, while Biden’s audiences have been noticeably smaller — a circumstance that was apparent over the weekend as well.

At a rally ahead of the Friday night fundraising dinner, Biden’s staff walked through the crowd with handfuls of extra tickets, searching for people to fill the seats they had purchased. At the dinner, hundreds of empty seats remained in six upper-level sections reserved by the campaign. Outside those sections sat boxes of unused inflatable noisemakers.

For all of the candidates, the weekend put into sharp relief what can happen to those failing to break through the crowded primary. O’Rourke’s departure came, his staff later said, after he decided he didn’t want to make some of the changes that Harris has been making, like cutting staff to finance commercials.

O’Rourke gathered with his supporters on the banks of the Des Moines River, promising to continue to fight for issues he cares about and urging them to stay engaged. He spent an hour comforting those who had believed in him.

“This is going to be good. I promise,” he said to one supporter. And to several others: “Keep the faith.” Several people told O’Rourke they were unsure whether they would find another candidate, and he assured them that they would. As he embraced one woman, the sound of her sobs mixed with the high-beat tempo of another candidate’s marching band nearby.

Inside the arena, O’Rourke’s campaign staff and volunteers abandoned their posts, leaving behind boxes of chant instructions — including “I-O-W-A, Beto’s going all the way” — that would never be passed out, a reminder of how quickly a candidate could vanish from the race.