On the heels of a major Iowa poll which showed a big slip in his big lead over the Democratic field, Joe Biden was conspicuous by his absence from the guest list for a big party event in the state on Sunday.

Nineteen candidates for the presidential nomination were due to give five-minute speeches at the Democratic Party Hall of Fame dinner in Cedar Rapids – among them leading contenders Bernie Sanders, Elizabeth Warren, Pete Buttigieg and Kamala Harris.

Biden, who was attending his granddaughter’s graduation, was not among them. He is due to visit Ottumwa, Iowa, on Tuesday – the same day Donald Trump will visit Council Bluffs and Des Moines.

In the poll released by CNN and the Des Moines Register on Saturday night, Biden led the 23-strong field in the early voting state by eight clear points. But he only polled at 24%, down from his usual 30%-plus.

Sanders was second with 16%, ahead of Warren with 15% and Buttigieg with 14%. Harris was the only other candidate above 5%, with half as much support as Buttigieg.

That’s a strong showing for Elizabeth Warren … there are people who are paying attention and that’s step one Ann Selzer

The former Texas congressman Beto O’Rourke and the Minnesota senator Amy Klobuchar were the only other candidates to get more than 1% support among likely caucus participants. Both attracted 2%. Higher-profile candidates attracting no support included New York senator Kirsten Gillibrand and New York City mayor Bill de Blasio.

The poll also showed that voters who identify as liberal now favour Warren, and that the primary concern among respondents is who is best placed to beat Trump.

“That’s a strong showing for Elizabeth Warren,” Ann Selzer, president of the eponymous company which conducted the poll, told the Register. “I think that all of the publicity lately and all of the polls lately are so Biden-heavy that for her to have any metric that shows her on par [with him] … it says to me there are people who are paying attention [and] in a field this big, that’s step one. First, you have to get people to pay attention.”

On Sunday, Sanders sought to deflect questions about his disagreements with Biden or the importance of any poll so far out from the caucuses, which will be held on 3 February next year.

“We’re not going to get 50% of the vote in Iowa,” Sanders told CNN’s State of the Union, comparing the contest to 2016, when he went toe-to-toe with the narrow winner, Hillary Clinton.

“I don’t think anybody will. I think we have an excellent chance to win here, we’re going to win in New Hampshire, and I think we have a very strong chance of being the candidate who will defeat the worst president in the modern history of this country, Donald Trump.”

The fact we can bring Texas and its 38 electoral votes with us shows that we are best prepared to take on Donald Trump Beto O'Rourke

One of the candidates with 2% in Iowa, Klobuchar, told CBS she could build support by being a midwestern senator “running on a track record of getting things done”.

“I’m clearly on the debate stage and expect to be there in the fall,” she said. “And I think that’s going to give opportunity to voters in Iowa and all across the country to really narrow it down.”

Her fellow two-percenter, O’Rourke, a charismatic candidate whose polling numbers have nonetheless gone into reverse, also played down the Iowa poll.

“If I relied on polls in any race that I’d run,” he told ABC’s This Week, “I never would have been able to serve in the United States Congress. We never would have tried to take on Ted Cruz.”

Beto O’Rourke stretches before the Pride Fest Fun Run 5K in Des Moines, Iowa on Saturday. Photograph: Scott Olson/Getty Images

That was a reference to the 2018 Texas Senate race which O’Rourke lost to a Republican incumbent. Asked why he thought he was the man to beat Trump, the former congressman focused on his roots in a strongly Republican state and his stance against the president on the hot-button issue of immigration.

Biden has so far focused on states where he polls strongly against Trump, including his native Pennsylvania and the post-industrial “rust belt”, where the president beat Clinton on his way to victory in the electoral college despite a near-3-million-ballot defeat in the popular vote.

In an interview with the Guardian published on Sunday, Bruce Haines, a former steel executive and Trump supporter from Northampton county, Pennsylvania, pointed to a main issue of contention in Democratic ranks nationwide.

Should the party pick a progressive with bold policy ideas, such as Sanders or Warren, or should it opt for Biden or O’Rourke, centrists with high name recognition in blue-collar or Republican-leaning states away from the liberal coasts?

“So I think support [for Trump] is going to strengthen as we get closer to the election,” Haines said. “And as we sort out who he’s going to run against – will it be a socialist, or will it be Biden?”