WEBSTER CITY, Iowa — Iowa’s presidential caucuses disenfranchise huge blocs of voters. The state is 91 percent white. It is not easy to get to, or get around in. But to a greater degree than in recent campaigns, this unrepresentative and idiosyncratic state is proving that it is the only electoral battleground that matters for Democrats from now until caucus night on Feb. 3.
Former Vice President Joseph R. Biden Jr.’s campaign said it would have 110 staffers in the state by the end of September. Senator Kamala Harris has promised to visit every week in October. Mayor Pete Buttigieg on Sunday launched a John McCain-like everything-on-the-record bus tour and is advertising on local television and radio. Senator Amy Klobuchar is 49 counties into her tour of all 99 in the state. And on Monday, Senator Bernie Sanders will begin a “Bernie Beats Trump’’ tour of eastern Iowa to highlight what he says is his strength as a general election candidate.
As a new poll suggested a significant shift in the primary’s top tier — with Senator Elizabeth Warren overtaking Mr. Biden for first place — candidates at the annual Polk County Steak Fry in Des Moines on Saturday tried to cut through the furor surrounding President Trump, Ukraine and Mr. Biden to deliver their message to voters still sifting through their preferences from one of the largest fields in history.
The candidates’ renewed sense of urgency has set the stage for a four-month sprint to a night of caucuses that remains the single biggest prize in American politics.
“We know that Iowa is where we can turn heads,’’ Mr. Buttigieg, of South Bend, Ind., said aboard his campaign bus Sunday. “Even the other early states will be looking at what Iowa did.”
The increasing focus on Iowa, where voters must attend an hours-long midwinter evening gathering to participate in choosing their party’s nominee, has come at the expense of New Hampshire, where the nation’s first primary comes eight days later.
No Democratic presidential candidate is betting their entire campaign on a strong performance in New Hampshire, meaning it may be the first time since 1984 that there is not a candidate focused solely on the Granite State in a contested presidential race.
With so many candidates running, much remains unsettled. And both Iowa and New Hampshire have a history of boosting underdog candidates, with John Kerry rising from the single digits to a caucus victory in 2004 and Bill Clinton declaring himself the “comeback kid” when he finished second in New Hampshire’s 1992 primary.
Yet those examples, like others often cited by this year’s back-of-the-pack candidates, took place before presidential campaigns had become entirely nationalized. Nowadays, Democrats at campaign rallies in rural Iowa are more likely to recite talking points they’ve heard on cable news or social media than something they read in their local newspaper.
Collectively, the 10 Democrats who appeared on the debate stage in Houston earlier this month have spent more than twice as much money on Facebook advertisements targeted to Iowans since June 29 than to people in New Hampshire, Nevada or South Carolina, according to data from Bully Pulpit Interactive, a Democratic political firm.
No candidate in the race has a bigger chasm between their online investments in the first two nominating states than Ms. Harris, of California, who since June 29 has spent $115,178 on Facebook ads in Iowa, but just $2,164 on those in New Hampshire.
After some grumbling that she had gone more than a month since last visiting the state, Ms. Harris was overheard last week saying she was “moving to Iowa,” with an expletive added for emphasis. Her team announced last week that it would add 60 new staff members and open 10 new offices, and she plans to return weekly until Halloween. Her campaign manager told reporters she was aiming for a “top three” finish in the state.
The historically large field has left even the candidates unclear how many Democrats will have a plausible claim to continue on to New Hampshire, Nevada and South Carolina.
Ms. Harris called a third place showing a goal but said she didn’t know if a failure to meet that threshold would doom her campaign.
“I honestly don’t know,” she said in an interview before a campaign stop Friday in Cedar Falls. “I think it will be a confluence of factors that ultimately decides how long anyone stays in and what that means.”
A new poll from the Des Moines Register and CNN, released Saturday night, showed a reordering of the Democratic field. For the first time in the Register’s poll this year, Ms. Warren, of Massachusetts, at 22 percent support, led Mr. Biden, who the survey found was the first choice of 20 percent of would-be caucus-goers. Mr. Sanders, of Vermont, placed third, at 11 percent, with Mr. Buttigieg at 9 percent and Ms. Harris at 6 percent.
“This is the first major shakeup” of the race, the Register’s pollster, Ann Selzer, told the paper.
The poll found that just one in five likely caucusgoers said they had made up their minds, and 63 percent could still be convinced to shift their support to someone else.
Ms. Klobuchar, of Minnesota, who was tied for sixth in the poll, said after presiding over the opening of a new campaign office Friday in Cedar Rapids that there may be more than twice as many tickets out of Iowa as usual.
“You could be number six,” Ms. Klobuchar said before the poll results were announced. “I’m really serious, in a 20-person field and people will go, ‘Not bad.’ They’re going to look at who is in the top eight.”
Mr. Buttigieg predicted a smaller number of candidates would survive Iowa.
“I don’t know how many people can emerge, I think that comes down to how many people voters can keep track of,” he said. “I’d be shocked if it’s more than five.”
The entire field is, at the moment, chasing Ms. Warren. She is drawing the largest crowds in the field — 2,000 came to see her Thursday night in Iowa City, two hours after and three miles away from a Harris stop in Coralville that drew about 200 to a local brewery. She declined on Sunday to take a victory lap over her new Iowa polling standing.
“I don’t do polls,” she said in Detroit, where she marched with striking auto-workers outside a General Motors plant. “We are still months away from the Iowa caucuses and the first primary elections.”
A cadre of challengers to Mr. Biden and Ms. Warren are augmenting their Iowa operations, adding staff, opening new offices, collecting endorsements and launching new fund-raising appeals as they aim to create the aura of momentum while building organizations to turn out supporters for the state’s Feb. 3 caucuses.
Ms. Klobuchar, from neighboring Minnesota, has support from more current and former Iowa state lawmakers than any other 2020 presidential candidate, according to Iowa Starting Line, a local political news website.
And Senator Cory Booker of New Jersey, who tied with Ms. Klobuchar in the Register’s poll, issued an S.O.S. to supporters this weekend, saying that without an infusion of $1.7 million by the end of the month his campaign may no longer be viable.
While Democrats elsewhere in the country, representing a national party that is increasingly diverse, offer quadrennial grumbling about Iowa, officials here aim to use their status to force the presidential candidates to focus on local issues.
J. D. Scholten, a Democrat running for Representative Steve King’s seat in a 39-county swath of northwest Iowa, said he will soon schedule campaign stops across his district with each of the presidential campaigns in towns of 1,000 people or less to test their appeal to rural voters.
“I really want to see the engagement,” Mr. Scholten said at Saturday’s Steak Fry, which drew a crowd of more than 12,000 people. “I think that’s where I’ll see. No polling can tell me, nothing like that. I want to see whether the people’s heads nod or not.”
Lisa Lerer contributed reporting from Cedar Rapids, Iowa, and Stephanie Saul from Detroit.