Scott Olson via Getty Images Supporters of Democratic presidential candidate Sen. Elizabeth Warren (D-Mass.) wait for her to arrive at the Iowa Democratic Party's Hall of Fame Dinner on June 9, 2019, in Cedar Rapids, Iowa.

CEDAR RAPIDS, Iowa ― Teagan Roeder, a 19-year-old student at the University of Iowa, was too young to participate in the 2016 Iowa caucuses. If he could have, he would have supported Sen. Bernie Sanders, he said.

But this time around, Roeder has eyes for a different candidate.

“I’m definitely doing Warren,” Roeder said excitedly Sunday. “I just feel like she’s more of the progressive for me.”

Here in Iowa, Sen. Elizabeth Warren appears to have started slowly eating into Sanders’ progressive base. A Des Moines Register poll released Saturday found that Warren, Sanders and South Bend, Indiana, Mayor Pete Buttigieg, have fallen into something resembling a three way-tie for second in the Democratic primary among likely Iowa caucus-goers, polling at 16%, 15% and 14%, respectively. The presumed Democratic front-runner, former Vice President Joe Biden, polled at 24%.

That’s a clear shift from March, when the Des Moines Register last released the results of its well-regarded poll. Back then, Sanders received the support of 25% of those polled ― just a few percentage points behind Biden ― while Warren pulled in 9% and Buttigieg received 1%.

The challenge Sanders faces in keeping hold of the progressive wing of the party is clear. When Sanders ran for president against Hillary Clinton in 2016, he did so as the sole face of the left wing of the Democratic Party. But this time around, numerous candidates have embraced elements of his progressive platform.

Sanders faces especially stiff competition from Warren, who has consistently made her own news through a series of ambitious policy proposals, including a wealth tax, universal child care and a plan to eliminate the vast majority of Americans’ student loan debt.

“There’s more of a spectrum of progressive choices in this race,” Roeder said.

On Sunday, Sanders, Warren and 17 other Democratic presidential candidates descended on Cedar Rapids to speak at the Iowa Democratic Party’s sold-out Hall of Fame celebration. Inside the event, Warren celebrated her team’s ambitious campaigning in the first months of the Democratic Primary.

“I’ve done 90-plus town halls, I’ve taken literally thousands of questions. I’ve been to 20 states and Puerto Rico. And yeah, we’re coming up on our 30,000th selfie,” Warren told the audience. “That’s how you build a grassroots movement.”

In Iowa alone, Warren has attended 36 events, spoken to more than 7,000 Iowans and taken almost 5,000 selfies over eight trips. She also employs more than 50 paid staff members in the state, which has garnered the attention of top local Democrats.

“There is no doubt that Elizabeth Warren has had a remarkable field effort,” said Sue Dvorsky, a former Iowa Democratic Party chair.

The strategy has started to translate into energy among Iowans. Prior to the Des Moines Register releasing its poll on Saturday, J.D. Scholten, a Democrat who ran against GOP Rep. Steve King in Iowa’s 4th Congressional District last year, said Warren seemed to be “sneakily doing a lot better than what the polling suggests.”

“It is common to hear people say things like ‘I’m not going to express an opinion but Elizabeth Warren makes it hard to not express an opinion,’” said Steven Drahozal, the chair of the Dubuque County Democrats in Iowa.

On Monday morning, outside an ethanol facility in Dyersville, Iowa, Warren told HuffPost it was “too early to be talking about polls.”

“But it has been a real honor to have the chance to be out and talk to folks all across Iowa about the kinds of changes we could make together that would make a real difference and touch people’s lives,” the senator said.

Sanders’ support in Iowa remains substantial. More than 12,000 people in the state have donated to the Sanders campaign, and 25,000 have signed up to volunteer, according to his campaign. About 10,000 people have attended his events here.

On Sunday, Sanders marched toward the Hall of Fame celebration alongside impassioned supporters and striking McDonald’s workers fighting for higher wages and union representation.

But Robert Shrum, a University of Southern California politics professor who has worked on numerous presidential campaigns, said it seemed Warren was taking votes directly from Sanders. While Sanders remains firmly seated in second place in the national polls, a Quinnipiac poll released last month showed Warren receiving 30% of the vote from very liberal Democrats, while Sanders pulled in only 22%.

“Elizabeth Warren has wonked her way into the top tier of candidates,” Shrum said.

While Roeder would be “perfectly happy” with either Warren or Sanders as the nominee, he said he’s come to appreciate the number of proposals Warren has put out.

“I just feel like Warren is more of the planner. And Bernie, while I love him for this, he’s definitely more of the idealist,” Roeder said.

On Sunday, some of the people who spoke to HuffPost outside the Hall of Fame event felt similarly.

Johnny Khuu, 24, was a Sanders supporter in 2016. But this time around, he’s decided to join Warren’s campaign as an organizing fellow.

“I feel like the way she laid things out is much more well thought out,” Khuu said. “She has no problem talking about how she wants to pay for things. In fact, she’s ready to go at it. Whereas Bernie kind of brushes that question off.”

Others remain conflicted and undecided.

Paul Lockard, 61, who teaches economics at a nearby college, said he’s waffling between Warren and Sanders, with Tulsi Gabbard a distant third.

“I like the fact that Elizabeth has very concrete plans that should be doable,” Lockard said. “Whereas some of what he says is more rhetorical.”

“I’m also concerned about the age label,” he said, before noting, “I’m still up in the air.”

Don Oliver, a self-employed painting contractor sporting a Sanders pin, also remains undecided.

“If he and Elizabeth Warren teamed up, like they should have done last time, they’d probably be leading the race right now,” Oliver said.

Warren, Sanders and the other Democratic candidates still have eight months until the Iowa caucuses. But as the event wrapped up Sunday evening, a woman who had been leading the Warren team in chants all day announced they were going to be wrapping up.

The volunteers seemed upset and yelled “No!”

They were just getting started.

This has been updated with additional comment from Warren.