Holding red and white signs, they protested outside Los Angeles Mayor Eric Garcetti’s election party on Tuesday, demanding the city take a tougher stand against deportation.

The next day, they rallied in support of the International Women’s Day strike, demanding social and economic equality for women.

These weren’t liberals. They were card-carrying members of the Democratic Socialists of America, one of the fastest-growing groups on the American left.

The surge of activism sweeping the U.S. since Donald Trump’s election has energized the nation’s largest socialist organization, which has tripled in size over the last year to claim more than 19,000 dues-paying members. That’s a record for the DSA, which was founded in 1982.

“People really felt that they had to do something to combat the incoming Trump administration,” said David Duhalde, deputy director of the Democratic Socialists of America’s national leadership, which helps coordinate chapters spread across 40 states. “We’re not only somebody you can resist Trump with, we’re somebody you can build a better world with.”

The grass-roots group forms only a small part of America’s swelling ranks of activists. The American Civil Liberties Union amassed hundreds of thousands of new members after Trump’s victory.

The fast-growing and liberal-centric Indivisible movement claims 4,500 associated groups compared with the 121 chapters of the Democratic Socialists of America. As far as political parties go, California alone boasts 8.7 million registered Democrats.

But unabashed socialism hasn’t had this big of a voice in American politics in decades, and many leftists say they feel energized. New members of the Democratic Socialists of America say they want to build a grass-roots movement engaged at the local level — and either pull the Democratic Party leftward or shove it out of the way.

That’s why, on election night, as Garcetti won one of the most commanding mayoral victories in Los Angeles history, dozens of socialists protested. A few of the group’s provocateurs infiltrated the well-dressed crowd of Democrats inside, where they shouted against U.S. Immigration and Customs Enforcement: “ICE out of L.A.!”

“If you’re gonna do it, have some fun,” said Josh Androsky, a 30-year-old stand-up comedian who co-chairs the Los Angeles chapter’s “agit-prop” committee and who joined after Trump’s election. “A large portion of our members were radicalized by the election and the Democrats failing over and over again.”

The Democratic Socialists of America’s membership increase seems driven by three factors: younger Americans, who polls say are more open to socialism than previous generations; the 2016 Democratic primary campaign of Sen. Bernie Sanders, a self-described democratic socialist whose race ignited a grass-roots following but also left bitter feelings about the Democratic Party; and the galvanizing effect that Trump’s election has had on left-leaning Americans.

Kevin Joerger, 24, of Los Angeles first got involved with politics when he volunteered for Sanders’ campaign, and when Trump won, “I had to do something more to stay sane,” he said.

Although he “rooted, sort of,” for Hillary Clinton to win the 2016 presidential election, the Democratic Party didn’t satisfy him. Joerger said he felt that big business had taken over politics and that capitalism had failed Americans individually, and he wanted “to be part of a movement in my community and see change locally, and not just nationwide.”

So he joined the Democratic Socialists of America.

Among leftists, the DSA is considered a “big tent” organization. Decisions are made by topic-specific committeesinstead of through adherence to rigid ideology, which allows for a relatively wider range of opinion than other groups. The group also takes a more incremental approach to reining in free-market capitalism.

“As we are unlikely to see an immediate end to capitalism tomorrow, DSA fights for reforms today that will weaken the power of corporations and increase the power of working people,” says the group’s website. “Our vision is of a society in which people have a real voice in the choices and relationships that affect the entirety of our lives.”

It’s yet to be seen what kind of effect the group might have. Socialism has never been a dominant force in American electoral politics. Previously, its most successful American leader was Eugene V. Debs, who won 6% of the presidential vote in 1912 running on the Socialist Party of America ticket.

Although many conservatives view the Democratic Socialists of America as subversive radicals, some leftists see them as not nearly radical enough.

“The farthest they can go is supporting elements such as Bernie Sanders,” Marc Wells, a Trotskyist, said disdainfully as he handed out leaflets for the World Socialist Web Site at the International Women’s Day strike in Los Angeles. The site is published by the International Committee of the Fourth International, which, like other Marxist groups, advocates harder-line approaches.

Duhalde, the DSA’s deputy national director, said the group is “flexible and willing to change” compared with other leftist approaches.

“There’s been a huge generational shift of millennials who are going to reinvent the socialist project,” Duhalde said, adding that more than half of members who joined since Trump’s victory are younger than 30.

Many new members say they heard about the group on Twitter, where Democratic Socialists of America members and supporters often put red rose emojis next to their user handles, an armband for the digital era. New enlistees have posted photos of their membership cards, which show their names along with the title “official socialist organizer.”

One of the group’s biggest online boosters is the actor and comedian Rob Delaney, star of the TV show “Catastrophe,” a Sanders supporter who regularly exhorts his 1.36 million Twitter followers to join the Democratic Socialists of America.

Like many DSA members, Delaney favors single-payer healthcare, in which the government covers healthcare costs, and he’s been impressed by the government-run National Health Service in Britain, where he films his show. He once carried $50,000 in medical debt in the U.S. following surgeries he needed after a car accident, even though he was insured.

“If you’re not lying to yourself you recognize that income inequality and systemic racism/misogyny work really, really well to keep the poor poor and make the rich richer. And that’s not OK to me,” Delaney said in private messages on Twitter.

“DSA’s ideas weave all that together in a very pragmatic and actionable way. And they’re fantastic organizers,” Delaney said. “DSA espouses and promotes these issues in a way I really, really like and believe is the best way to defeat Trump and the GOP.”

matt.pearce@latimes.com