DES MOINES, Iowa — Eli Newson is fired up about flexing her electoral muscle for the first time in a presidential race — that is, as long as Sen. Bernard Sanders is in the 2020 mix.

Otherwise, she’s out.

“If it is not Bernie Sanders in this election … then that is the end of electoral politics for me,” Ms. Newson said over the weekend, adding that she would shift her focus to “other forms of activism.”

The 19-year-old college student is part of a new wave of “Bernie-or-Bust” activists who came of voting age in the aftermath of the bitter 2016 primary battle between Mr. Sanders and Hillary Clinton that splintered the Democratic Party.

That has left the likes of Ms. Newson and Evan Sand, 20, with little faith in establishment politics and hardened their shared belief that the 78-year-old senator is the only candidate who can be trusted to deliver on his promise of bold systemic change.

“If you are not for ‘Medicare for All,’ you are not for canceling student debt, abolishing ICE — like there has to be a standard,” Mr. Sand said. “If we keep voting for the establishment candidate or a candidate that has a watered-down message from Bernie’s, there will never be change.”

For his part, Mr. Sanders has vowed that if he does not win the nomination he will rally behind the Democratic nominee — and plenty of his supporters here say they plan to follow his lead.

Mr. Sanders rode the hunger for a political evolution to within a whisker of defeating Mrs. Clinton in the 2016 Iowa caucuses. He went on to clobber her in New Hampshire’s first-in-the-nation primary, but ultimately finished second in a nomination race that was marred by leaked emails showing party officials favored Mrs. Clinton.

Four years later, the left-leaning Vermont senator has not only held on to much of his 2016 support, he’s added some of this cycle’s most passionate and outspoken activists. They celebrate Mr. Sanders’ antipathy toward Wall Street and what supporters see as a career-long fight for the working class.

He’s also lending credence to the adage that what doesn’t kill us makes us stronger.

More than three months after suffering a heart attack, Mr. Sanders is leading the field in the most recent Des Moines Register poll and is running strong among young voters and first-time caucus-goers in Iowa.

Randy Roose, a supporter of the more moderate Pete Buttigieg for the nomination, attributed Mr. Sanders’ popularity with younger generations to the far-left, big-spending proposals that Mr. Roose worries will balloon the federal budget, saddling future generations with more debt.

“Bernie is the candyman — you know, the candyman can,” the 68-year-old said. “There is still a real world out there and it has to be paid for.”

But those backing Mr. Sanders say they are unshakably committed.

The Register poll found 59% of those who name him as their top choice said they have made up their minds completely, compared to 48% for Sen. Elizabeth Warren, 44% for former Vice President Joseph R. Biden Jr., and 40% for Mr. Buttigieg, a former mayor of South Bend, Indiana.

“Bernie seems to be a little bit like Trump,” said Tom Courtney, co-chair of the Des Moines County Iowa Democrats. “He has this following that never goes away.”

“I think he could walk down the street and shoot somebody and they would stick with him just like Trump says about his supporters,” Mr. Courtney said.

Some Democrats are openly concerned about what the pro-Sanders forces will do if the Vermont senator falls short again. Democrats say the party cannot afford a repeat of 2016 when all the infighting made it hard for the party to come together against then-candidate Donald Trump.

“There are a lot of Democrats who are still resentful of the whole Bernie-thing from 2016 and it is irritating,” said Susan Chronister, who is backing Ms. Warren.

Ms. Chronister said she wants Mr. Sanders’ backers to know that she has every intention of supporting him if he wins the nomination — so much so that she felt the need to drive home the point to Sanders’ volunteer who recently knocked on her door.

“I said it two times because I wanted to make sure he heard me: ‘If Bernie is going to be our nominee I will vote for him,’” she said. “I didn’t say this, but I wanted to: I sure hope that if he is not [the nominee] that the Bernie supporters will vote for whoever the [nominee] is because we have got to.”

Bill Nellans shared a similar concern. He said he plans on backing whoever is the nominee but has been a bit thrown off by the way his 23-year-old daughter and her boyfriend have gone about backing Mr. Sanders.

“They are kind of bad-mouthing now both Elizabeth Warren and Biden, you know,” the 65-year-old said. “I’m like, ‘Come on you guys, cut it out.’”

“I think that is the biggest reason Hillary didn’t beat Trump in ‘16 was because the Bernie supporters pushed all through this process to bad-mouth Hillary,” he said.

It is unclear how deep the “Bernie-or-Bust” sentiment runs within the base of the party, but it has been on display, including outside a recent Biden campaign stop where Ms. Newsom and Mr. Sand greeted voters funneling out of the event with shouts of “abolish ICE” and “Bernie 2020.”

“Personally I don’t see a lot of difference between a Biden 2020 presidency and a Trump 2020 presidency because what matters is not necessarily the person that is in that position, but the structural oppression that has been in place for the United States for decades and only someone as committed and as forthright as Sen. Sanders to actually start to take that down,” Ms. Newson said. “No other candidate is going to actually be able to do that.”

Mr. Sand, who volunteered for Mr. Sanders’ 2016 campaign as a high school student, said Mr. Trump is a “racist” and “bigot,” but said after the DNC “rigged” the 2016 election he doesn’t feel like he owes the party his vote, which also will be his first in a presidential election.

“The Democratic Party needs to realize that the time is now and they need to stand behind a candidate who can beat Trump and a candidate who is igniting working-class voters to actually participate in the system that they feel disenfranchised from,” Mr. Sand said.

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