Supporters of Democratic presidential candidate Bernie Sanders packed his campaign's Colorado headquarters Sunday as staffers and key endorsers promised that the Vermont senator's grassroots organization will lead to a repeat win in the state's Super Tuesday presidential primary.

"We are going to win this fight," said Pilar Chapa, the Sanders campaign's state director. "It's a long battle, and this is a rare opportunity for us. We get to elect someone who stands for the values that we believe in."

More than 300 Sanders backers converged for an opening celebration that was part rally and part organizing event inside the storefront office building in Denver's Capitol Hill neighborhood, just over three weeks before mail ballots for the March 3 primary start going out to Colorado voters.

Sixteen Democrats are on Colorado's presidential primary ballot, including Sanders' fellow front-runners Joe Biden, Elizabeth Warren and Pete Buttigieg, as well as billionaire Mike Bloomberg, who has been flooding the state with advertising and is hiring dozens of staffers.

Sanders won Colorado's precinct caucuses in 2016 with roughly 60% of the vote to Hillary Clinton's 40%, but this year's semi-open primary — unaffiliated voters can cast ballots in either major party's primary for the first time — and crowded contest for the Democratic nomination could scramble the elements that led to one of Sanders' biggest wins last cycle.

"The progressive movement has grown, and it's grown up here in Colorado," said former state Rep. Joe Salazar, the Thornton Democrat who was among the first elected officials in Colorado to endorse Sanders in 2016.

"In 2016, we were laughed at, the progressive movement. We were laughed at, we were chided, we were made fun of at the caucus. The establishment didn't think that there was a real movement happening," he said.

Recalling how he "threw down" at that year's state convention, and Sanders emerged with a larger share of delegates than his vote share on the night of caucuses, Salazar said: "Here we are, four years later, and we have the same candidate, who doesn't shift based on the political winds, who's saying the same things he said four years ago, that other candidates are trying to attach themselves to. We have him leading in states where he needs to lead, and he's coming to Colorado to whoop some ass."

After narrowly losing the 2018 Democratic nomination for state attorney general to Phil Weiser, Salazar was named executive director of Colorado Rising, an organization opposed to fracking.

At Sunday's office opening, the pugnacious Salazar boasted that he'd just received an award from the Democratic Party, despite "all those naysayers back in 2016, who didn't think this movement would survive."

"It's to show the establishment, it's to show those people who believe in incrementalism — who are moderates — that not only are we here, we are the backbone of the party," he said, drawing sustained cheering. "And if you don't want to listen to us, there will be consequences to that. Because the true Democrats are in this room."

The tension between the progressive wing of the Democratic Party and the "billionaires, corporations and the candidates bankrolled by billionaires and corporations" was on display throughout the event, with nearly every speaker contrasting Sanders' "people-powered" grassroots support with his opponents, including fellow Democrats.

"We are willing to stand up to them and say, 'Your days are over with, and we are coming,'" Salazar said, jabbing his finger in the air.

Chapa, a former executive director of the Colorado Democratic Party, urged Sanders supporters not to abandon the Democrats.

"We have to infiltrate the party," she said. "If you look at the platform, the Democratic platform, they have had to follow Bernie's lead."

Denver-based journalist David Sirota, a longtime Sanders confidante and a senior adviser and speech writer with the national Sanders campaign, told the crowd to brace for "an intense, unprecedented set of attacks on Bernie Sanders to try to stop him from winning the nomination" as the primary enters its next phase.

Sanders' detractors, Sirota said, "are terrified — and they should be terrified, because they can see the polls that show their nonsense, the attacks they have been throwing at us, are not working."

He added: "It's not working, because we've built an unprecedented grassroots campaign."

Tim Dickson, the state field director, told Colorado Politics that around 15,000 volunteers have signed up to work for Sanders in Colorado. Through this weekend, he said, they've organized more than 500 voter-contact events, including meet-and-greets, phone banking, canvassing and the campaign's "very data-driven friend-to-friend program," that builds voter lists from volunteers' contacts.

Chapa noted that the Colorado Sanders campaign expects an influx of dozens of organizers arriving from Iowa and New Hampshire after those states finish voting in mid-February.

"Bernie's not kidding when he says this is a grassroots effort," Dickson said. "We have more volunteers than anyone here in Colorado."

Supporter Leslie Eiler of Woodland Park said she's putting together several organizing events every week in heavily Republican Teller County.

"It's a challenging environment, but we have Berners up there," she said as she piled her arms with yard signs and a T-shirt to give away.

State Rep. Emily Sirota of Denver, who was endorsed by Sanders in 2018 before unseating a moderate Democrat in a primary, said she was thrilled to see Sunday's turnout.

"People are always looking to discount Bernie and underestimate him," she said after posing for a snapshot with her husband, David, and a pair of Sanders fans. "Then you see the incredible number of people, this diverse working-class, racially diverse, generationally diverse coalition of people who are here because they believe in a government that works for the people."