The Bernie Sanders campaign plans to knock on half a million Iowa doors in January. Elizabeth Warren’s team is asking supporters for spare couches to house out-of-state volunteers. And the national polling leader, former vice president Joe Biden, claims a sharp uptick in volunteer sign-ups as he has secured high-profile endorsements in the state.

After a year of campaigning, and with less than a month to go before the first and therefore most important single contest in the Democratic nomination fight, few if any are confident of the outcome. At least four candidates are seen as having a shot to win Iowa, or, alternatively, to suffer a crippling result that could hobble their campaigns going forward, especially if there is a late surge by a lower-tier contender.

The result, according to interviews with top campaign strategists and local Democratic officials, is a hotly contested sprint to the Feb. 3 caucuses — a struggle that could either propel a clear winner into the next-voting states with momentum or open a months-long fight for the delegates needed to secure the party’s presidential nomination. Unlike past primaries, several of the top candidates are expected to have the financial resources and dedicated fan base to wage long campaigns even if they finish in the middle of the pack in Iowa.

“We have a jump ball,” said Jeff Link, a strategist who worked on Barack Obama’s winning 2008 Iowa campaign. “They are all kind of right there, and I think we have yet to see the big shifts. My hunch is that 75 percent of people, even if they say they have a candidate, could change their mind.”

The lack of clarity has been accentuated by a campaign calender that soon could be upended by the scheduling of the Senate impeachment trial of President Trump, which could pull three of the top candidates back to Washington to perform their role as jurors, and possibly force a rescheduling of the next Democratic debate in Des Moines on Jan. 14.

A dearth of public state polling results since mid-November, when former South Bend, Ind., mayor Pete Buttigieg led a bunched field, also has added to the confusion, as the campaigns fly partially blind. Voters face what strategists describe as the electoral equivalent of a chicken-vs.-egg problem: Iowans want to pick the candidate best able to defeat Trump but are holding off picking a candidate until it becomes more clear who others are backing.

“Electability, while it was important in 2007, is far more important now,” said Paul Tewes, a top Iowa strategist for Obama, referring to the lead-up to the 2008 Iowa result that galvanized support around the young senator from Illinois. “So people are going to weigh that up to the last moment.”

The situation is made more complicated by a late entry into the race, former New York mayor Mike Bloomberg, who is promising to spend hundreds of millions of dollars to win votes starting in March, long after Iowa decides, in delegate-rich states such as California, Texas and Florida. It’s a prospect that could effectively punish the early states for failing to produce a definitive result.

Amid a barrage of campaign ad spending, Iowa party leaders have begun to express concern that an inconclusive outcome — or the selection of a candidate who goes on to fizzle in subsequent states — could jeopardize Iowa’s position as the location of the nation’s earliest nominating contest, a point of pride and a source of tens of millions of dollars for the local economy.

“All of us are getting a little worried,” said Sean Bagniewski, chair of the Polk County Democratic Party, who says the hope is for Iowa to repeat the role it played in 2008 with Obama’s momentum-building victory. “People really want us to have another Obama moment. We can feel the DNC is very actively considering the idea of letting Iowa lose its first-in-the-nation status.”

In the absence of clear data, campaigns have been relying on their internal statistics — crowd counts, volunteer hours, caucus commitments, office locations and staff.

There are signs of hope for some, such as the recent turnout for Buttigieg and the renewed energy behind Sanders, whose campaign appears to have fully recovered following the candidate’s October heart attack.

“We have a lot of anecdotal accounts of people showing up to our events saying they have never caucused before,” said Brendan McPhillips, Buttigieg’s Iowa state director. “People are making up their minds, but it also still is very fluid.”

Candidates with less polling support continue to hold events across the state in the hope of breaking into the top tier, including two of the remaining nonwhite candidates in the race, businessman Andrew Yang, who keeps attracting strong crowds, and Sen. Cory Booker (D-N.J.), who has vowed to continue his campaign despite missing the qualification thresholds for the last Democratic debate. Sen. Amy Klobuchar (D-Minn.) has seen sharp increases in her still relatively small crowds in recent weeks, campaign advisers say, raising hopes that she can break into the top three or four finishers, effectively dethroning one of the top-tier candidates.

“The people of Iowa are really starting to come through,” said Justin Buoen, Klobuchar’s campaign manager. “When we do well in Iowa, we are going to be well positioned to do well in New Hampshire, Nevada and beyond.”

Advisers to Biden, who has received the endorsements of Rep. Abby Finkenauer (D-Iowa) and former governor Tom Vilsack, have credited part of the rising energy to renewed concern about Democrats finding a candidate to beat Trump. Biden’s polling lead in hypothetical matchups with Trump, which are not always predictive this early in a cycle, have become a central part of the campaign’s message.

Warren, who has faded from her fall stature as a clear leader in the race, is still widely perceived to have perhaps the deepest and most experienced grass-roots operations, and rival campaigns have recently credited Biden, who struggled through the summer in the state, as having righted his ship in the state.

Advisers to Sanders, who narrowly lost the 2016 Iowa caucuses to Hillary Clinton, remain convinced he has the largest volunteer and donor list in the state. And like Warren, he is not holding high-dollar fundraising events, a burden that will keep other candidates out of the state for much of the month.

Buttigieg, for example, is not scheduled to return to Iowa until Jan. 12, allowing him to attend big fundraising events in states such as New York, Colorado and California.

“There is no Emmy Award for best organizing. The award is who wins,” said Ari Rabin-Havt, the deputy campaign manager of the Sanders effort. “There is no Academy — there is a vote.”

Starting in 1976, the top individual vote-getter in the Iowa caucuses has gone on to win the Democratic nomination seven out of nine times. The exceptions are 1988, when third-place finisher Michael Dukakis went on to win the nomination, and 1992, when eventual nominee Bill Clinton decided not to contest the state.

At their core, the Iowa caucuses function more like a focus group than a general election, with a small subset of deeply engaged and well-informed voters making the decision for everyone else.

In a state of more than 3 million residents, the most optimistic turnout projections for Feb. 3 would get close to 300,000 to give up their Monday night for hours of deliberations. In a five-way race, that means the winner probably will claim no more than 2 or 3 percent of the state’s population.

Assuming four or more candidates clear the 15 percent threshold required to win delegates, the Iowa outcome is unlikely to give any candidate a meaningful delegate advantage, because the state’s 41 pledged delegates to the party’s national nominating convention set for July in Milwaukee represent just a fraction of a percent of the total vote and will be mostly divided in separate buckets, based largely on congressional district results.

Each campaign has been gaming out privately what claims to momentum it and its rivals will need from a mixed result to succeed in subsequent contests.

Sanders, who has demonstrated a clear floor of support and high fundraising all cycle, is seen by his rivals as a likely factor deep into the primary calendar, even if he performs poorly in the first contest. The stakes are higher for Warren and Buttigieg, who committed heavily to the state in hopes it would propel them to later contests.

The Biden campaign has argued from the start that it can win the nomination even if it loses the first two contests, but in recent weeks the campaign has become more bullish on his performance in Iowa. A person familiar with the Biden’s campaign thinking said the candidate is less worried about Buttigieg winning in Iowa, because the campaign believes he will not be able to make up ground among black and Latino voters, who have heavily favored Biden in polling.

Strategists say they have been surprised by how fluid voters in Iowa have been in moving between candidates with very different ideological profiles. Though there have been substantive clashes around issues such as Medicare-for-all and how expansive to make new college tuition subsidies, with Sanders and Warren representing the party’s liberal wing and Biden, Buttigieg and Klobuchar tending to stake out more moderate positions, many voters have not defined the race this way.

“There are Biden-Bernie voters. There are Biden-Buttigieg voters. There are Warren-Buttigieg voters,” said Pete Kavanaugh, Biden’s deputy campaign manager. “A lot of observers think this is about progressive candidates versus moderate candidates, but voters don’t seem to agree.”

Another unusual strategic conflict has been playing out between Buttigieg and Warren, who ended September with nearly the same amounts in their bank accounts and bifurcating strategies. Over the next five weeks, when Warren played defense on health care and started to drop in the polls, Buttigieg flooded the airwaves, spending nearly $1.5 million on ads that were unanswered by Warren, according to data provided by Democratic strategists.

“It was an igniter for him when a lot of other people were not up in a significant way,” said Jim Margolis, an advertising consultant for Sen. Kamala D. Harris (D-Calif.), who dropped out of the race after Thanksgiving.

Though Warren is now roughly matching Buttigieg’s spending on the airwaves, Margolis said the early television spend, which casts a wider net than the grass-roots organizing, could also benefit candidates later in the process, because the exact universe of caucusgoers can change, especially in high-turnout years.

“What we have learned in ’04 and ’08 and ’16 is that there are an awful lot of people who show up at caucuses who nobody ever saw before,” he said. “They weren’t on anybody’s list.”

Chelsea Janes contributed to this report.