© Copyright 2020, Des Moines Register and Tribune Co.

DES MOINES, Iowa – Sen. Bernie Sanders leads the Democratic field three weeks ahead of Caucus Day in Iowa – narrowly overtaking his closest competitors, who remain locked in a tight contest just behind him.

A new Des Moines Register/CNN/Mediacom Iowa Poll shows 20% of likely Democratic caucusgoers name Sanders as their first choice for president.

After a surge of enthusiasm that pushed Pete Buttigieg to the top of the field in November, the former South Bend, Indiana, mayor has faded, falling 9 percentage points to land behind both Sanders and Sen. Elizabeth Warren. Warren is at 17%; Buttigieg, 16%; and former Vice President Joe Biden, 15%.

“There’s no denying that this is a good poll for Bernie Sanders. He leads, but it’s not an uncontested lead,” said pollster J. Ann Selzer, president of Selzer & Co., which conducted the poll. “He’s got a firmer grip on his supporters than the rest of his compatriots.”

2020 candidates on the issues:Here's where they stand on gun violence, immigration, health care and more

The poll of 701 likely Democratic caucusgoers was done Jan. 2-8 and has a margin of error of plus or minus 3.7 percentage points.

Biden, Buttigieg, Sanders and Warren have remained clustered atop the Register’s Iowa Poll throughout the 2020 campaign cycle, though no one has definitively pulled away from the pack. Instead, each of the top four has now led the Iowa Poll at some point this cycle as the field continues to shift.

The percentage of those who say their mind is made up about which candidate to support on caucus night has risen to 40% – up 10 percentage points from November. Forty-five percent say they have a favorite candidate but could be persuaded to support someone else and an additional 13% have not picked a favorite candidate.

► More:'I am one of the undecided': With a month to go, many likely caucusgoers still unsure about their choice

“The caucus process is an invitation to keep an open mind,” Selzer said, noting that the Iowa Poll produced similar numbers of undecided caucusgoers at the same point during 2016's crowded Republican primary.

Other Democratic candidates – including those banking on a late burst of momentum – failed to gain much ground in the January poll. Sens. Amy Klobuchar of Minnesota and Cory Booker of New Jersey held steady, Klobuchar at 6% and Booker at 3%. Entrepreneur Andrew Yang grew his support from 3% in November to 5% today.

Linda Ireland, a 65-year-old poll respondent from Waterloo, said Booker is her first choice, and Klobuchar is her second. But she also likes Buttigieg and Yang.

"I am going to caucus for Booker, and I might move over to Amy or even Pete," said Ireland, a retired writer and copy editor.

The caucus process makes those second choices crucial. After a first vote on caucus night, candidates who don't receive at least 15% in a precinct get dropped from contention there. The people who initially backed a candidate who failed to earn 15% then get to vote again for a final time. So Ireland is considering all her choices.

"If Pete’s doing really well, and there’s no one else viable in my precinct, I would go over to him. I would probably go down the line: Klobuchar, Yang and then Pete," she said.

In the new poll, a slew of other contenders fell well below the 15% viability mark.

Rep. Tulsi Gabbard of Hawaii, who is focusing her attention on New Hampshire, and entrepreneur Tom Steyer are at 2%. Steyer is holding steady in Iowa but has gained enough ground in other early states to earn a place on Tuesday's debate stage.

Former New York City Mayor Michael Bloomberg, who is skipping the four early-voting states and compensating for his late entry into the race by purchasing tens of millions of dollars’ worth of TV ads elsewhere, barely registers here. His support fell a percentage point from 2% to 1%.

Support for former Sen. Michael Bennet of Colorado and former Rep. John Delaney of Maryland rounded to 0%. Though Bennet has largely foregone competing in Iowa in the race's closing weeks, Delaney – who was the first candidate to begin campaigning here in 2017 – has committed to a massive 40-stop RV tour of the state this month.

Massachusetts Gov. Deval Patrick and author Marianne Williamson, who ended her campaign after the poll was completed, were not named as a first choice for president by a single poll respondent.

'There's just the Bernie factor'

Could Sanders gain the Iowa caucuses win that eluded him in 2016? Selzer said the data suggest it’s entirely plausible.

“There’s just the Bernie factor,” Selzer said. “Which is (even) stronger than we saw in the last cycle.”

Although this is the first time in his back-to-back races for president that the Vermont senator has led the Iowa Poll, his support this cycle has remained remarkably firm, a fact his campaign has worked to exploit.

Sanders' supporters are more likely than those who support the other leading candidates to say their minds are made up (59%), and they are “extremely” enthusiastic about their candidate (49%). Just 32% of Warren's supporters describe themselves as extremely enthusiastic, and 26% each for Biden and Buttigieg.

“There was a thought that his support was a holdover from when he ran before and that that would evaporate,” Selzer said. “It certainly has not evaporated."

Sanders on the issues:The 2020 candidate shares his views on student debt, Medicare for all, and the Green New Deal

She said Sanders is holding on to many of those who caucused for him in 2016 while also growing support among young and first-time caucusgoers. Those are notoriously difficult groups to turn out on caucus night, she said, but Sanders has done it before and appears to be on track to do so again.

He has retained support from 44% of those who say they caucused for him in 2016. Warren earns 20% of his former supporters.

And he leads the field with those younger than 35, earning 36% of their support; Warren follows him at 20%.

"He’s someone I followed in the last primary and really liked," said Matthew Quick, 25, a P.E. teacher from Essex. As a teacher, Quick said Sanders' focus on education and improving teacher quality and pay stands out.

"When he put his name in the hat in 2020, I knew I was going to be in all the way," he said.

Buttigieg fades, but many still considering him

Buttigieg held a commanding lead in the Register's last Iowa Poll, in November, at 25% – 9 percentage points ahead of Warren, his closest competitor. But in the weeks since, he has faced more criticism over his record, his moderate policy positions and his inability to win over voters of color nationally.

Buttigieg fares worst among the leading candidates in the share of supporters whose minds are made up: 40%. That's compared with the 59% of Sanders' supporters, 48% of Warren's and 44% of Biden's.

Buttigieg's favorability ratings also have gone in the downward direction. The share of those who say they view Buttigieg favorably has fallen 4 percentage points from November to 68% today. The share of those viewing him unfavorably has risen from 16% to 24%.

Pete Buttigieg on the issues:Here's where Buttigieg stands on gun violence, immigration, health care and more

"That used to be his great claim to fame: He was very likable, and there were very few who didn’t like him," Selzer said. "Some of that luster is lost in this poll."

Still, he retains a wide footprint of support in Iowa. As he did in November, Buttigieg tallied the largest universe of potential supporters, with 60% saying they are considering him in some capacity. That includes the 16% who name him as their first choice for president, the 15% who say he is their second choice, and 29% who are actively considering him.

That narrowly bests Warren, who is still the first or second choice or is being actively considered by 59% of likely Democratic caucusgoers – falling from 66% in November and a high of 71% in September.

Biden fell 3 percentage points in this footprint measure, to 55%, and Sanders ticked up 1 point to tie him. Klobuchar and Yang follow with 41% and 40%, respectively.

Selzer said the numbers seem to indicate a little less room for Klobuchar and Yang to grow.

“They seem to be settled on the number they’re going to get,” she said. “With this big of a field and with four strong top contenders, what sort of showing in the caucus would be good enough for Andrew Yang or for Amy Klobuchar to move forward?”

Seventy-eight percent of men under age 45 say they view Yang favorably, as do 73% of white men with a college degree.

Klobuchar gets more favorable ratings from older likely Democratic caucusgoers, including 69% of women age 55 and older. Conversely, just 32% of caucusgoers younger than 35 say they view Klobuchar favorably.

Data show progressive, moderate 'lanes'

Selzer said she typically eschews the idea of political “lanes,” arguing that caucusgoers don’t approach their decisions that way.

“But you look at these data and you kind of go, ‘Well, these look like lanes,’ ” she said.

Sanders and Warren are battling for progressive caucusgoers, and Biden and Buttigieg are attracting a more moderate crowd.

Higher proportions of Biden and Buttigieg supporters – 75% and 72%, respectively – say choosing a candidate with the superior chance of defeating President Donald Trump is extremely important to them, personally. It’s 54% among Sanders supporters and 53% among Warren supporters.

Seventy-eight percent of Biden's supporters and 84% of Buttigieg's supporters say that a candidate’s ability to unite the country is extremely important in their choice. That factor is extremely important for 65% of Sanders' supporters and 58% of Warren's.

And 64% of Biden supporters and 61% of Buttigieg supporters say having a candidate who can bring Republicans and independents on board is extremely important. Just 38% of Sanders' supporters and 46% of Warren's supporters feel that way.

Further, a plurality of Biden's supporters, 29%, say Buttigieg is their second choice; a plurality of Sanders' supporters, 44%, say Warren is theirs; and a plurality of Warren's supporters, 31%, say Sanders is their second choice. Buttigieg's supporters are split on their second choices, with 20% saying Warren and 18% saying Biden.

“This poll really displays the Sanders-Warren connection and the Biden-Buttigieg connection,” Selzer said.

What are candidates' strengths, weaknesses?

As the United States faces heightened tensions in the Middle East and with Iran, supporters of Biden and Buttigieg express more confidence in their ability to lead the military than the supporters of Sanders or Warren place in their candidates.

Among his own supporters, 86% say Biden's ability to lead the U.S. military is more of a strength than a weakness. Just 1% say it's more of a weakness for the former vice president. On the campaign trail, Biden regularly touts his foreign policy acumen, arguing he's ready to take over the role of commander in chief on Day One.

Buttigieg, who is the only candidate polling in the top four to have served active duty in the military, also fares well: 87% of his backers say his ability to lead the military is a strength, and just 7% call it a weakness.

Foreign policy:Here's where the 2020 candidates stand on tensions with Iran and North Korea

"I think it’s good to have somebody who at least has the context of that mindset," said poll respondent Christina Powers, a 36-year-old stay-at-home mom from Dexter who names Buttigieg as her first choice for president. "You definitely have to be able to build bridges and understand the way the military thinks. … And I think, also, because we’ve been at war for half of his life, he probably understands it’s something that is not something to crave – that it’s not a good thing."

Even among their own supporters, Sanders and Warren struggle on this measure: 61% of Sanders' supporters say his ability to lead the military is a strength, and 23% call it a weakness; for Warren, 58% of supporters say her ability to lead the military is a strength, and 29% call it a weakness.

Sanders' supporters are also concerned about his health.

The 78-year-old senator had a heart attack earlier this year, and although he hasn't slowed his campaign schedule, 49% of his supporters say his physical health is a weakness, while 37% call it a strength.

Quick, the poll respondent and Sanders supporter, said he believes Sanders' age comes with experience. But he said he worries Trump could weaponize Sanders' age and physical health.

"I feel like it’s something the Republicans could try to use against him," Quick said.

Although Biden is nearly as old, at 77, far fewer of his supporters – just 28% – say his health is a weakness. A majority, 55%, say it's a strength.

Warren is the other septuagenarian among the top-tier candidates. The 70-year-old is often seen running onto stages and through events, and 86% of her supporters say her physical health is a strength.

Buttigieg, who would be the youngest president ever elected, leads on this measure: 95% of his supporters say his physical health is an asset, not a weakness.

► MORE ON THIS POLL: Read our methodology

About this poll

The Iowa Poll, conducted Jan. 2-8, 2020, for the Des Moines Register, CNN and Mediacom by Selzer & Co. of Des Moines, is based on telephone interviews with 3,131 registered voters in Iowa, including 701 who say they will definitely or probably attend the 2020 Democratic caucuses.

Interviewers with Quantel Research contacted randomly selected active voters from the Iowa secretary of state’s voter registration list by telephone. The sample was supplemented with additional phone number lookups. Interviews were administered in English. Responses for all registered voters were adjusted by age and congressional district to reflect their proportions among active voters in the list.

Questions based on the sample of 701 voters likely to attend the 2020 Iowa Democratic caucuses have a maximum margin of error of plus or minus 3.7 percentage points. Questions based on the sample of 3,131 registered voters have a maximum margin of error of plus or minus 1.8 percentage points. This means that if this survey were repeated using the same questions and the same methodology, 19 times out of 20, the findings would not vary from the true population value by more than plus or minus 3.7 or 1.8 percentage points, respectively. Results based on smaller samples of respondents — such as by gender or age — have a larger margin of error.

Republishing the copyright Iowa Poll without credit to the Des Moines Register, CNN, and Mediacom is prohibited.

Brianne Pfannenstiel is the chief politics reporter for the Register. Reach her at bpfann@dmreg.com or 515-284-8244. Follow her on Twitter at @brianneDMR.

Support reporters who follow your government by becoming a subscriber. Sign up at DesMoinesRegister.com/Deal.