CEDAR RAPIDS, Iowa — Bernie Sanders spent three days hopscotching across Iowa, from a college auditorium to an LGBTQ pride festival to a Democratic jamboree, where he made a new case against presidential rival Joe Biden.

Elizabeth Warren has blanketed the state with some 50 staffers courting supporters with kaffeeklatsches, happy hours and trivia nights. During her eighth visit this year, which concludes Monday, she refreshed her contrast with the former vice president, as a new Iowa survey showed her surging.

And Pete Buttigieg whisked around Iowa presenting himself as a leader from a new generation and warning against reverting to the “old normal” that, he seemed to imply, was associated with Biden. Buttigieg is preparing to ramp up his modest ground operation in the state, according to a campaign adviser.

The battle in all-important Iowa to emerge as the main alternative to Biden, who sits atop the polls here and nationally, has kicked into high gear. Nineteen candidates for the Democratic nomination blitzed the state over the weekend, including the senators from Vermont and Massachusetts, and the South Bend, Ind., mayor.

Notably missing from the fray was Biden himself, who campaign advisers said skipped a major Democratic event Sunday event to attend his granddaughter’s high school graduation. He plans to come to Iowa on Tuesday, setting up dueling appearances with President Trump, who intends to fly in the same day.

Biden has conducted his campaign so far as if he’s competing largely against the president, ignoring the team of opponents hoping to supplant him for his party’s nomination. That and Biden’s limited footprint in Iowa have created an opening for an underdog in the first caucus state, local Democrats say.

“There’s a lot of ruffled feathers,” said Polk County Democratic Chairman Sean Bagniewski about Biden’s approach so far.

Iowa has a history of rewarding upstarts and serving as a treacherous challenge for more established contenders. Barack Obama’s 2008 victory and Sanders’s near-win in 2016 — both coming against early favorite Hillary Clinton — are the most recent examples.

“ ‘Presumptive nominee’ are lethal words,” said Jerry Crawford, a prominent Iowa Democratic attorney who recently endorsed Sen. Cory Booker (N.J.). “I would never want them attached to anyone I’m supporting.”

The state has also been a powerful launchpad. The last Democratic nominee who did not win the Iowa caucuses was Bill Clinton in 1992 — a year when many candidates took a pass because one of the state’s senators, Tom Harkin, also was running for president. Although the 2020 primary calendar has been reshuffled, with California moving up its vote, many strategists feel there is no substitute for the momentum created by performing well in the state that reports its results first.

These factors present a dangerous dynamic for Biden — who launched his campaign later than most of his rivals. His team plans to deploy 50 full-time staffers in Iowa this month, advisers said. Bagniewski, who has not endorsed a candidate, described Biden’s current Iowa operation as “very much a skeleton crew at this point.”

Biden is also navigating the toughest stretch of his campaign so far. He has faced criticism over his team lifting language without credit and using it in his education and climate proposals. And as challengers grow more overt in taking him on, his abrupt reversal last week on a law restricting the use of taxpayer money for abortions has drawn widespread scrutiny.

A Des Moines Register-CNN-Mediacom Iowa poll released Saturday night showed Biden leading the Democratic pack with 24 percent, but Sanders (16 percent), Warren (15 percent) and Buttigieg (14 percent) are bunched up within striking distance. Warren and Buttigieg have improved their standings, the poll indicated, while Sanders has slipped.

During the past few days, some of Biden’s top opponents have sought to seize the moment in Iowa and try to build on earlier investments in the state. Sanders, who fought Clinton nearly to a draw in 2016, warned that a cautious Democratic platform could have serious electoral consequences.

“That approach is not just bad public policy, but it is a failed political strategy that I fear could end up with the reelection of Donald Trump,” Sanders said Sunday at the Iowa Democratic Party’s Hall of Fame Celebration, a marquee event where 18 other candidates also delivered short speeches. His comments were seen as an implicit attack on Biden’s candidacy.

The remarks marked a further escalation by Sanders against Biden, a more centrist figure who differs with the liberal senator on some major policy issues. Sanders has been critical of Biden’s support for sweeping free-trade deals and the Iraq War. A week ago in California, he condemned “middle ground” policy ideas and appeared to ding Biden for not showing up at the Democratic state convention at which Sanders spoke.

The Vermont senator was in Iowa on Friday, Saturday and Sunday, hosting town halls, sitting for a live podcast interview, rallying with McDonald’s workers for higher wages and greeting supporters at the Pride festival.

The Sanders campaign recently added 20 paid staffers to its Iowa team, bringing its total to over 40, estimated Misty Rebik, the Iowa state director for the Sanders campaign. The campaign is placing an emphasis on training and activating volunteer organizers to help win over caucusgoers, Rebik said.

“We’re hyper, hyper-focused on getting back in touch with our base, working with them on their skill-building, making sure that they have what they need in their own community to be successful at directing their own voter contact,” Rebik said. She noted that 25,000 Iowans have agreed to volunteer for Sanders.

Sanders has struggled to sustain the enthusiasm he generated when he launched his campaign earlier this year. The new Iowa poll is in line with other indicators suggesting that Warren’s rise and Biden’s entrance have put a squeeze on Sanders.

Warren appeals to many of the same left-leaning voters drawn to Sanders. Like Sanders, she has started contrasting herself with Biden more aggressively, with implied critiques of his worldview.

In her speech at the Hall of Fame gathering on Sunday, she was critical of candidates who raise money from big donors. Biden has aggressively raised money from wealthy patrons, some of whom have previously given to Warren.

“I’m not spending my time with high-dollar donors and with corporate lobbyists,” said Warren. “I’m spending my time with you. That’s how we build a grass-roots movement in America.”

Many Democratic officials and strategists in Iowa say that Warren has the most robust ground operation in the state, rivaled only by Booker’s. She has hired 50 staffers in Iowa, including 30 organizers.

The organizers host a regular schedule of community events across the state that put them in face-to-face contact with potential caucusgoers. They include meet-and-greet happy hours, coffee chats and pub-style trivia nights such as one held in Perry last week.

At that event, Morgan Sperry, a 23-year-old Warren organizer, mingled with local Democrats at adjacent tables with piles of Warren stickers on them as they ate tacos, sipped beer and cider, and mulled trivia questions about the town’s history. A volunteer who traveled from Massachusetts brought cookies bearing the word “Persist,” a signature Warren slogan.

“Keep people engaged, show that we are present in the communities, that we’re investing in them,” said Sperry, describing the goal of her events. She said she is responsible for three counties; in addition to trivia, she hosted two house parties and three or four coffee hours last week, she said.

Beyond Warren and Sanders, Buttigieg has also become a threat to Biden’s grip on Iowa. At a house party in Winterset on Friday, Buttigieg, who is 37, gay and a military veteran, stood on a porch and pitched himself as the candidate the current moment demands — “something completely different.”

He warned that Trump could win a second term if Democrats “look like we are offering more of the same,” and if their message is “let’s go back, let’s just rewind a little bit.” Buttigieg declined to comment directly on Biden when reporters asked about him afterward.

Buttigieg’s challenge in the coming months will be to translate his growing popularity into a more vigorous campaign in Iowa. He has just five paid staffers on the ground right now, according to a campaign adviser, and is aiming to quadruple that number by the end of June.

But the South Bend mayor has some competition to be the youthful, fresh-faced alternative to Biden. Former Texas congressman Beto O’Rourke, 46, is making a similar pitch to voters. In an interview after a town hall in Ottumwa on Friday, O’Rourke sought to distinguish himself from Biden, who has said that four years of the Trump presidency “will go down in history as an aberration.”

“I think it’s really important that we acknowledge that we can’t just go back to what this country was like pre-Donald Trump,” said O’Rourke, when asked if he sees any contrasts between himself and Biden. “We need really bold, very ambitious leadership.”

Some Iowa Democrats said they’ve detected recent growth in O’Rourke’s operation. The campaign has 44 staffers on the ground, 37 of whom are on the organizing team, said Norm Sterzenbach, O’Rourke’s state director. Still, O’Rourke was at just 2 percent in the latest Iowa poll.

Other campaigns are stuck in neutral in Iowa — or worse. Booker, who is running on a platform that emphasizes unity and love, holds just 1 percent in the latest survey, despite having a significant campaign operation in the state.

Sen. Kamala D. Harris (D-Calif.), who has walked a line between the party’s liberal and moderate wings, has been slow to build her Iowa campaign, perplexing some local Democrats. She intends to have more than 65 Iowa staffers on July 1, according to her campaign, and plans to visit the state three times over five weeks, starting Sunday. She placed fourth in the latest poll, with 7 percent, after drawing big crowds during her late winter post-announcement foray into the state.

Woodbury County Democratic Chairman Jeremy Dumkrieger said candidates who are not in the top tier can still compete in Iowa if they put in some face time.

“You couldn’t do this in California or New York,” he said, where campaigns will be conducted largely via television and social media.

Challenging Biden’s ideas and record are also going to be key if the others are going to whittle his lead in the polls, the chairman added.

“They have to draw the contrast,” Dumkrieger said. “If they agree with everything he said, then they wouldn’t have a shot, right?”