Just hours before presidential candidates will pitch themselves to Iowa Democratic activists in the first-in-the-nation caucus state, a new poll of likely caucusgoers is rattling the race — and hinting that a formidable front-runner is not as invincible as he might appear.

The Iowa Poll, conducted by veteran pollster Ann Selzer for The Des Moines Register and CNN, found 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 leading the Democratic field with 24 percent of the vote.

The race for second place is a statistical tie between 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.) at 16 percent, 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.) at 15 percent 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 at 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.) is the only other candidate who registers significant support. She clocked in at 7 percent of the vote. Six percent of voters said they were not sure who they would choose on caucus night.

Eight months before voters head to their caucus sites, though, the poll shows movement within a Democratic primary that is still wide open.

Here are six takeaways from the Iowa Poll.

Biden's support is shaky

In December, months before he even entered the race, nearly a third of Iowa voters said they backed Biden. Today, about six weeks after he announced he would run, Biden's support has fallen by a third.

Biden's declining support came even before this week's controversy over his flip-flop on the Hyde Amendment, which came after Selzer began fielding her poll.

In December, 82 percent of Iowa Democratic voters said they saw Biden favorably; this month, that number is down to 72 percent of Democratic voters, a 10-point drop. His unfavorable ratings are up 9 points.

And Biden's backers are less enthusiastic about their chosen candidate than supporters of other candidates. Just 29 percent of Biden's supporters say they are extremely enthusiastic about backing the former vice president. An average of 43 percent of Sanders, Warren and Buttigieg supporters say they are extremely enthusiastic about backing their chosen candidate.

Sanders has a ceiling

Three years ago, Sanders came within a handful of votes of achieving a stunning upset over front-running former Secretary of State Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE. Today, almost every Iowa Democrat has an opinion of Sanders; only 5 percent of likely caucusgoers say they do not know enough about him to have formed an opinion, a lower number than everyone but Biden.

But Sanders also has one of the highest unfavorable numbers among Democratic candidates; a quarter of state Democrats say they view him negatively, about the same percentage as those who see Biden unfavorably.

The dichotomy between Sanders's high name recognition and his relatively low poll numbers suggest Sanders fans from 2016 are looking elsewhere this year — and that the 77-year-old self-avowed democratic socialist has a ceiling through which he cannot break.

As Warren, Buttigieg, Harris and the others introduce themselves to voters and earn more support, they are likely to pull from a population that backed Sanders over Clinton in 2016.

The high unfavorable ratings that Sanders and Biden suffer aren't the worst in the field. More than 4 in 10 Iowa Democrats see New York City Mayor Bill de Blasio Bill de BlasioOVERNIGHT ENERGY: California seeks to sell only electric cars by 2035 | EPA threatens to close New York City office after Trump threats to 'anarchist' cities | House energy package sparks criticism from left and right EPA threatens to close New York City office after Trump threats to 'anarchist' cities New Year's Eve in Times Square to be largely virtual amid pandemic MORE unfavorably.

Warren is on a growth trajectory

Selzer is considered one of the most skilled pollsters when it comes to surveying Iowa's electorate. What makes her especially effective is that her surveys consistently illustrate who's on the move, up or down.

And though it is eight months before the caucuses, Warren is the candidate on the move. She has a higher net favorable rating than any other candidate in the field; 71 percent see her favorably, while just 17 percent see her unfavorably. That's a better ratio than Biden, Sanders and Buttigieg.

An equal number of Democratic voters, 61 percent, say they are actively considering or backing Warren and Biden, the highest rates in the field. That's 5 points higher than those who say Sanders is on their ticket and 9 points higher than both Buttigieg and Harris.

Warren has been more aggressive in laying out detailed policy proposals than any other candidate. She has hired more staffers in Iowa than any other candidate, and she has spent more time in Iowa — more than two weeks — than any other front-runner.

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In terms of sheer growth, no one has improved more than Buttigieg, who wasn't even included in the December survey. Buttigieg scored just 1 percent support in the March survey, fielded about a week before his first CNN town hall, where he captured the Democratic electorate's attention and vaulted from also-ran to top contender.

The Beto bust?

No one is in a deeper slump than 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). In December, a month after O'Rourke narrowly lost a spirited challenge to Sen. Ted Cruz Rafael (Ted) Edward CruzCruz blocks amended resolution honoring Ginsburg over language about her dying wish Trump argues full Supreme Court needed to settle potential election disputes Press: Notorious RBG vs Notorious GOP MORE (R), 11 percent of Iowa Democrats said he was their first choice, while another 12 percent said he would be their second choice.

In this month's poll, only 2 percent of Iowa Democrats say Beto is their first choice, an 80 percent drop. Just 4 percent say he is their second choice.

The number of Iowa Democrats who say they view him unfavorably stands at 21 percent, almost double the number who said they saw him unfavorably back in December, while his favorable rating — 54 percent — is up only a single point. O'Rourke has held 52 events over 17 days, according to a tracker maintained by The Des Moines Register, but all that hard work isn't paying off.

Sens. Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (D-N.J.) and 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.) haven't gained any traction either. In Selzer's March poll, Booker and Klobuchar each claimed 3 percent of the vote; this month, Klobuchar is at 2 percent and Booker is at 1 percent, tied with the likes of former Rep. John Delaney John DelaneyCoronavirus Report: The Hill's Steve Clemons interviews Rep. Rodney Davis Eurasia Group founder Ian Bremmer says Trump right on China but wrong on WHO; CDC issues new guidance for large gatherings The Hill's Coronavirus Report: Kansas City Mayor Quinton Lucas says country needs to rethink what 'policing' means; US cases surpass 2 million with no end to pandemic in sight MORE (D-Md.), Washington Gov. Jay Inslee Jay Robert InsleeBarr asked prosecutors to explore charging Seattle mayor over protest zone: report Bottom line Oregon senator says Trump's blame on 'forest management' for wildfires is 'just a big and devastating lie' MORE (D) and tech entrepreneur Andrew Yang Andrew YangDoctor who allegedly assaulted Evelyn Yang arrested on federal charges The Hill's Campaign Report: Biden weighs in on police shootings | Who's moderating the debates | Trump trails in post-convention polls Buttigieg launches his own podcast MORE (D).

The best news for Booker: He's the second choice candidate for 6 percent of Iowa Democrats, a level of support that puts him in the conversation.

A tough field to poll

What's harder than keeping an Iowa voter on the phone while a pollster reads a list of all two dozen candidates running for the Democratic primary nomination? How about accounting for the two different kinds of elections Democrats will run next February?

The Iowa Democratic Party for the first time plans to hold what it is calling virtual caucuses — events in which voters who can't attend their polling places in person on February 3 can still make their preferences known.

Party rules say those who participate in virtual caucuses will be given the power to decide 10 percent of the delegates allocated during the caucuses, regardless of how many people show up in person or by phone. About 28 percent of likely Democratic caucusgoers said they would participate in the virtual caucus, making their votes worth about a third of those who actually show up in person.

That has forced Selzer to ask Iowa Democrats not just who they plan to support but how they plan to participate — and to weight support accordingly.

"Those who say they are likely to choose the virtual option are younger, more moderate, and more likely to be currently registered as 'no party,' which of course they must change to participate in the Democratic Party caucuses. They are also less committed to caucusing than those who say they intend to show up on caucus night," Selzer wrote in an explanation of her methodology.

Biden's support is higher among those who plan to caucus virtually, 33 percent, than those who plan to caucus in person, 23 percent. Buttigieg and Sanders fans disproportionately plan to show up in person. Warren's supporters are about evenly split, while Harris backers are more likely to say they will caucus virtually.

The new virtual caucus, meant to allow more voters to participate in the process, is likely to expand the universe of those who get to make their voices heard — and it's also going to make the process of measuring those voters all the more difficult.

Iowa Poll has clout

The Iowa Poll will be the lead story in tomorrow's Des Moines Register. It is already playing across CNN, the poll's other top sponsor. And it comes just as Iowa Democratic activists head to Cedar Rapids for the state party's annual Hall of Fame dinner, where 19 out of 24 presidential hopefuls will speak.

Selzer's poll will be the talk of the afternoon — so many candidates are speaking that the state party will kick off the event at 2 p.m. Central time. And it won't go unnoticed among state Democrats that the front-runner Biden is the only top contender who won't show up to woo the activist class.