Sen. Bernie Sanders waves to cheering supporters during the 2015 Iowa Democratic Wing Ding at the Surf Ballroom in Clear Lake, Iowa.

Aug. 14, 2015 Sen. Bernie Sanders waves to cheering supporters during the 2015 Iowa Democratic Wing Ding at the Surf Ballroom in Clear Lake, Iowa. Melina Mara/The Washington Post

The overflow crowds showing up to hear Bernie Sanders these days are a testament not only to his current popularity and the campaign’s social-media savvy but also to the promotional abilities of an alchemy of like-minded interests: progressive activists, labor unions and even Sarah Silverman.

The comedian took to Twitter to let her nearly 6.7 million followers know she would be at a rally for the Democratic presidential hopeful here Monday. That event drew an estimated 27,500 people — about five times as large as any crowd that has turned out for Democratic front-runner Hillary Rodham Clinton.

“Bernie always seems to be on the right side of history,” Silverman told the boisterous audience at the Los Angeles Memorial Sports Arena, noting that the 73-year-old was a civil rights activist in the 1960s, supported gay rights in the 1980s and strongly opposed the Iraq war before most other Americans.

[To his old socialist allies, Sanders has sold out]

All told, Sanders has attracted more than 100,000 people to his rallies in recent weeks, riding a wave of Facebook shares, retweets and old-fashioned word-of-mouth to become by far the biggest draw on the campaign trail.

Democrat Bernie Sanders's presidential campaign says the candidate drew 27,500 people to a venue in Los Angeles on Monday, after about 28,000 attended an event in Portland, Ore., on Sunday. (Sarah Parnass/The Washington Post)

Such turnout is no guarantee that Sanders will perform well in the crucial early-nominating states — fellow Vermonter Howard Dean preached to similarly large and frenzied audiences in mostly liberal enclaves in 2003, only to collapse as the Iowa caucuses approached.

But it is drawing energy and attention away from Clinton — whose aides peg her largest crowd to date at 5,500 — and exposing a lack of enthusiasm for her candidacy among some facets of the Democratic Party. And it is creating a network of small-scale donors and volunteers that could provide Sanders with the resources he will need to compete with the former secretary of state and first lady in the weeks and months ahead.

Roughly 28,000 people showed up for a recent Sanders rally in Portland, Ore. The self-described democratic socialist drew 15,000 in Seattle; 11,000 in Phoenix; 10,000 in Madison, Wis.; 8,000 in Dallas; and 4,500 in New Orleans.

[The Sanders predicament: Where do you fit all those people?]

This weekend, Sanders will host a couple of town hall meetings in Iowa and attend events with other candidates. His campaign has not yet said where his next large-scale rally will be.

In Los Angeles, Sanders’s campaign estimated that 27,500 people were jammed inside and outside the 16,000-seat arena — a figure impossible to independently verify. Nearly every seat appeared to be taken, and the arena floor was packed. Outside, thousands more watched the rally on large screens.

The throngs were greeted by Dante Harris, the leader of a flight attendants union local that had helped promote the rally.

A large crowd responds enthusiastically to Sen. Bernie Sanders’s rally in Los Angeles. (Charles Ommanney/The Washington Post)

“Did all of you make the trip out here on a Monday night because everything is going well for you and your family?” he asked.

“No!” the crowd roared.

Harris nodded and shouted back. “Workers’ lives matter! . . . Black lives matter! . . . The truth matters!” Struggling to be heard over the ensuing ovation, he said, “We can build a movement with Bernie!”

The audience was noticeably more diverse than those at recent Sanders rallies in Portland, Seattle and other majority-white cities. Los Angeles is majority-minority, with about 44 percent of its population Latino.

[Sanders needs to court black voters. Now he is.]

There were young hipsters and graying hippies. Some wore black T-shirts with red hammers and sickles; others wore black T-shirts that read “Black Lives Matter.” They sang along as the loudspeakers blasted songs by Willie Nelson, Tracy Chapman and Neil Young. One man carried a handmade sign that said “Bernie: Our Only Hope for Change.”

About a week before each Sanders rally, his campaign sets up a Web page announcing the location and blasts out an e-mail to supporters in that geographic area, asking them to RSVP. There are no paid advertisements.

The events are promoted on Facebook, with the campaign enlisting progressive groups and other “friends and allies” to help spread the word, said national field director Phil Fiermonte. From there, things tend to take on a life of their own.

Like Harris, climate-change activist Joe Galliani got a speaking slot in Los Angeles because his union had spread the word about the rally. He earned some of the loudest cheers as he denounced construction of the Keystone XL oil pipeline and touted the benefits of solar roofs.

Then there was Maria Barrera, the 31-year-old leader of the Coalition for Humane Immigrant Rights of Los Angeles, who tearfully noted that Congress has not enacted comprehensive immigration reforms since the 1980s. “For the last 25 years, families have been separated,” Barrera said.

[Activists drove Sanders from a stage, 15,000 people awaited him at the next]

The turnout and enthusiasm have been similar at Sanders events across the country.

For the Portland rally, a local group lobbying for a $15-per-hour minimum wage in the city — a cause Sanders supports nationally — drove a sizable contingent of people to the event. In addition, the Oregon Democratic Party sent an e-mail to people in its database letting them know about the event (with the disclaimer that the party is not taking sides in the presidential primary).

Local Democrats who attended the rally said it appeared to draw lots of newcomers to party politics.

“I hardly ever go to these events without running into friends, and I didn’t see many people I knew,” said Sue Hagmeier, the communications officer for the Democratic Party in Multnomah County, which includes Portland.

Hagmeier said she probably received 15 to 20 digital notifications in advance of the event, counting forwarded e-mails and Facebook posts. Sanders supporters also posted fliers on telephone poles and promoted the event by “chalking the sidewalks,” a tradition in a city known for big political rallies. (In 2008, an estimated 75,000 people came to see then-Sen. Barack Obama on the banks of the Willamette River as he closed in on the Democratic presidential nomination.)

Sanders is striving to harness the energy to help him in states such as Iowa and New Hampshire, where his crowds have been much smaller but still relatively robust. Volunteers hand out donation envelopes to rally attendees and carefully take down their contact information so they can be solicited for money later.

In Los Angeles, anyone who gave on the spot received a Sanders campaign T-shirt. Speakers asked the crowd to text “Bernie” to a five-digit number. In reply, they would receive text messages seeking money and volunteer time.

When Sanders came onstage, the cheers were deafening. His voice hoarse, the senator told the crowd, “This campaign is not a billionaire-funded campaign — it is a people-funded campaign.”

“There is no president who will fight harder to end institutional racism,” he said. “Or for a higher minimum wage. Or for paid parental leave. Or for at least two weeks of paid vacation. . . . Whenever we stand together, when we do not allow them to divide us up by the color of our skin or our sexual orientation, by whether a man or a woman is born in America or born somewhere else, whenever we stand together, there is nothing, nothing, nothing we cannot accomplish.”

“Bernie! Bernie! Bernie!” the crowd shouted in response.

Mike Jelf, 69, said he spent the day before the rally leafleting for the campaign in nearby Torrance. He brought his Saint Bernard, Munro, to the arena, wearing a shirt that said “Saints for Sanders.”

“I want to see a country that’s returned to the people, rather than a plutocratic oligarchy,” Jelf said. “I would like to have a planet that’s habitable for future generations.”

Jean-Luc St. Pierre, 19, said he flew from Maryland to attend the rally. He started a “Baltimore for Bernie” Facebook group targeting people in the home town of former Maryland governor Martin O’Malley, another Democratic presidential candidate.

Gloria Rios, from Eagle Rock, Calif., said she joined Sanders’s e-mail list a month ago. “I feel very strongly about him,” she said, adding that she has concerns about Clinton.

What are those concerns? She paused. For a really long time.

“There’s — too much conflict around her,” Rios said. “I’m the kind of person who feels that what you say and do has to match up.”

Sanders, she added, “resonates more closely to me.”

After the rally, the Sanders campaign tweeted a photo showing some of the thousands who had watched from outside the arena.

“Apologies to the large crowds who couldn’t fit indoors at tonight’s rally,” the tweet said. “We’re gonna have to get bigger venues!”

Wagner reported from Washington.