Former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE remains the top choice among Iowa Democrats, but his lead is slipping as two challengers build support in the crucial first-in-the-nation caucus state.

Biden is the first choice of nearly a quarter of Iowa Democratic caucus-goers, according to a gold-standard survey released Saturday night by the pollster Ann Selzer.

Selzer, who conducted the Iowa Poll for the Des Moines Register and CNN, found three contenders fighting for second place: Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.) clocks in at 16 percent support; Sen. Elizabeth Warren Elizabeth WarrenDimon: Wealth tax 'almost impossible to do' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context Democrats shoot down talk of expanding Supreme Court MORE (D-Mass.) won support from 15 percent of Iowa voters; and South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegThe Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice Hillicon Valley: FBI, DHS warn that foreign hackers will likely spread disinformation around election results | Social media platforms put muscle into National Voter Registration Day | Trump to meet with Republican state officials on tech liability shield MORE took 14 percent.

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Sen. Kamala Harris Kamala HarrisHarris faces pivotal moment with Supreme Court battle Nearly 40 Democratic senators call for climate change questions in debates Joe Biden has long forgotten North Carolina: Today's visit is too late MORE (D-Calif.) scored 7 percent of the likely caucus-goers. Former Rep. Beto O'Rourke Beto O'RourkeJimmy Carter says his son smoked pot with Willie Nelson on White House roof O'Rourke endorses Kennedy for Senate: 'A champion for the values we're most proud of' 2020 Democrats do convention Zoom call MORE (D-Texas) and Sen. Amy Klobuchar Amy KlobucharSocial media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Minn.) each earned support from 2 percent of those surveyed. Every other candidate in the crowded field scored at one percentage point or below.

"There is clearly a top tier, with Biden leading, and Sanders, Warren, and Buttigieg wrestling for second place," Selzer told The Hill. "Kamala Harris is on the bubble."

A little less than eight months before Iowa Democrats hold their caucuses, the absolute levels of support each candidate wins is less important than the trajectory their numbers show. Those trajectories are positive for Warren and Buttigieg — and distinctly negative for Biden and Sanders.

The survey, conducted June 2-5 among 433 likely Iowa Democratic caucus-goers, is the third time Selzer has tested the field. In December, she showed Biden leading with 32 percent, ahead of Sanders's 19 percent. In March, Biden led the field at 27 percent, while Sanders took 25 percent.

Today, Biden's support has slipped by a third from that initial survey. Sanders, whose 2016 campaign makes him among the best-known contenders in the field, has seen his support drop by more than a third.

Biden's supporters are less enthusiastic about his candidacy than supporters of every other candidate, Selzer said, a worrying sign for a front-runner who has suddenly appeared vulnerable.

By contrast, Warren won 8 percent support in the first poll and 9 percent in the second poll. Buttigieg wasn't even tested in the December survey, and by March only 1 percent of Iowa Democrats picked him as their favorite.

"The Warren campaign operation seems to be helping her gain traction," said Jeff Link, a Democratic strategist in Iowa who is unaligned in the 2020 presidential race. "This is a very encouraging poll for Mayor Pete."

O'Rourke has also taken a precipitous slide in recent months. In Selzer's December survey, 11 percent of Iowa Democrats backed the former Texas congressman. That number fell to 5 percent by March, and to just 2 percent Saturday.

The poll measures the strength of the field at a moment when many Iowa Democrats are only beginning to tune in. Nineteen candidates will travel to Cedar Rapids on Sunday to stump before the Iowa Democratic Party's annual Hall of Fame dinner, the first real Iowa cattle call of the race for the 2020 nomination. Every significant candidate, except for Biden, will address party activists.

The Iowa Poll carried a margin of error of plus or minus 4.7 percentage points for those planning to attend in-person and 7.6 points for those who plan to attend virtually.