SIOUX CITY, Ia. — Ted Benson left the Orpheum Theater feeling the same way he did when he arrived earlier in the evening: convinced that U.S. Sen. Bernie Sanders is the best candidate to challenge President Donald Trump in 2020.

Benson, a 41-year-old grain farmer from Hawarden, supported the 77-year-old senator in 2016 and has no intentions of wavering now.

"I will not look at other candidates," he said. "There’s no way I won't vote for him."

Nine months ahead of Iowa's first-in-the-nation caucuses, Sanders already has amassed a list of 24,000 committed volunteers — many of them 2016 holdovers — while many other candidates are still working to introduce themselves to Iowans.

It's a milestone that took Sanders' 2016 campaign months to reach and gives his 2020 effort a head start. But it also highlights a challenge unique to Sanders in this race: While others are working to build enthusiasm, he is working to retain and harness the momentum from his last campaign.

"We inherited 24,000 people who, in the last few months, have said, 'I'm still with Bernie,'" said Sanders' senior Iowa adviser, Pete D'Alessandro, who oversaw the senator's 2016 Iowa campaign. "Now, we have to keep them. We know how the caucuses work — our job is to give them something to do, but also keep them invested, keep them positive about the campaign."

At events small and large, Sanders' supporters sport T-shirts from the 2016 campaign. They hold copies of his books like churchgoers clutch Bibles. And they frequently interrupt his stump speech with shouts of, "That ain't right," as Sanders rails against the influence of the ultra-rich and criticizes the nation's health-care system.

"If somebody shows up, they probably know what they’re getting into," Benson said of the crowd.

D'Alessandro said the campaign is far ahead of where it was at this point in the 2016 cycle. And every day, he thinks about how to stay ahead. At rallies and town halls, he said the level of enthusiasm for Sanders looks and feels much like the last go around.

He pointed to the crowd of more than 250 who showed up for a town hall in early April in Malcom, a town of about 285 people between Iowa City and Des Moines.

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"I know they weren’t all from Malcom. But that means a lot of people got in their car and said, 'Let's go to Malcom,' which is probably the first time they ever said that," D'Alessandro said. "So you've got to have some kind of enthusiasm to do that."

At the Downtown Farmers' Market in Des Moines on Saturday, an unannounced visit from the senator approached mayhem as aides and volunteers worked to manage the crowd that instantly swarmed him.

Sarah Larson and her 6-year-old son unexpectedly bumped into the clutch surrounding the senator and quickly pulled him in for a hug and a photo.

"I am a huge supporter," she said, trembling with excitement afterward. "I'm crying because I just met him. Oh my God."

Larson, 30, is a stay-at-home mom and freelance baker in Urbandale who has supported Sanders for years. She likes that his policy stances have remained consistent over the years and described him as someone who "sticks to his guns."

"He's a very incredible human being, and you can tell he really cares about people and making changes," she said. "He gets our vote."

Superfans a staple of Sanders events

Sanders isn't the only presidential candidate to fuel fanaticism.

California entrepreneur Andrew Yang has developed a small but fierce following, particularly online, of fans who call themselves the "Yang Gang."

Those who go see Massachusetts Sen. Elizabeth Warren regularly deck out in apparel bearing the catchphrase, "Nevertheless, she persisted" — an homage to the words used by Senate Majority Leader Mitch McConnell when he cut short her speech during a debate on the nomination of Sen. Jeff Sessions of Alabama as attorney general.

In mid-April, Pete Buttigieg was greeted in Des Moines by a much larger-than-expected crowd of 1,600 people — including one woman who held a handmade sign showing the South Bend, Indiana, mayor's two dogs, Truman and Buddy.

But Sanders superfans are endemic at his Iowa events.

His audiences occasionally contain a few curious caucusgoers shopping the field. But more often, a Sanders event looks more like church for the faithful than a sales pitch to the skeptical.

Barbara Kelley says she's been "totally obsessed" with Sanders' so-called political revolution since his last run. But, she said, it's not about his personality — it's about his policies.

"For the first time in my life, I really threw myself into politics," said Kelley, a radio DJ and producer in Davenport. "I'm a 60-year-old 'Bernie bro.'

A self-described idealist, she says her excitement even drove her to move to Iowa to caucus for him in 2016. After a relationship broke apart in Texas, she chose to come to Iowa both to be closer to her daughter and to support Sanders in Iowa.

"It's kind of a 50-50 thing," she said.

Kelley served as a Sanders state delegate in 2016. She phone-banked for hours at a time during that cycle. And this year, she's stayed busy on Twitter, criticizing other candidates and defending Sanders.

"I think Bernie's going to win," she said. "I think he will be the next president. I will stake my life on it."

Keeping the faithful while growing the coalition

Sanders knows he must work to keep people like Kelley on board. But he also knows that he needs to grow his coalition.

"Our job, obviously, is to retain as many of the folks that we had last time and to get new ones," he said in an interview with the Des Moines Register. "And I think the theme of this campaign is going to be a little bit of a variation of what it was last time."

Sanders has already expanded his message with a renewed focus on rural America and the state of agriculture. On Sunday, he rolled out a new set of policies calling for "radical" changes in American farming and agribusiness.

"I come from a rural state," Sanders told the Register. "It's an issue we probably should have talked about more last time."

At his Malcom town hall, one man, desperate to be called upon, yelled out that he had traveled all the way from Alabama to see Sanders.

Sanders tapped him for the last question. The man, wearing a vintage jersey of NFL great Barry Sanders, said he was a huge fan, but asked Sanders whether he would support Yang's universal basic income plan, which would give the country's adults direct monthly cash payments from the federal government.

"Nah, I got a better idea," Sanders said.

"You won't do it?" the man asked incredulously.

"Nope," Sanders said. "Sorry you came all the way from Alabama."

He told the crowd that he was familiar with Yang's idea, but he prefers to create millions of new jobs through a $1 trillion infrastructure plan that would rebuild bridges and airports, invest in green energy and weatherize old homes and buildings.

That rejection didn't seem to faze the man, who smiled as Sanders answered. And he told the senator he still wanted a picture with him before he left.