The biggest city in the country is led by a democratic socialist, and another is in the running to lead the second biggest. Seattle has a socialist mayor. And in 2027, a democratic socialist will almost certainly be taking the reins of the nation’s capital.

With her convincing victory in the Democratic primary in Washington last week, Janeese Lewis George, 38, became the latest candidate to claim victory with the once-forbidden “S word” in her biography and an ambitious left-wing agenda, promising to harness the power of municipal government to tackle the costs and challenges of urban living.

Tapping into frustrations about housing and the cost of raising children, Ms. Lewis George pledged to greatly expand child care assistance, build tens of thousands more homes and expand rent stabilization. Her critics derided those promises as unrealistic; voters ate them up.

“I think people were like, ‘I don’t buy that the status quo is all we can do,’” Ms. Lewis George said in an interview. Instead, she said, they thought, “‘I want to see leaders do something more than tell people what they can’t do.’”

Ms. Lewis George, who in a city as blue as Washington is close to a lock in the general election, joins a vanguard of young democratic socialists, including the new mayors of New York City and Seattle. Some are formal members of the organized Democratic Socialists of America, some not, but all have won on platforms of robust government action, arguing that the older Democratic establishment has failed.

Democratic socialists say that solutions to challenges like the rising costs of child care and housing lie in community organizing and direct government action, not the free market or timeworn tax incentives. While they cast themselves more in the mold of a mayor from Stockholm than Leningrad, they do not shy from confrontation with business interests, whether that means private utilities or landlords, oligarchs or plutocrats.

Not everyone running from the left in big blue cities has won, as losers of the most recent mayoral races in San Francisco and Philadelphia can attest.

But socialist success indicates an ascendant left — a generational movement as much as a political one — might have considerably more room to run.

“We’re seeing real opportunities open up here,” said Kurtis Hagans, chairman of the D.S.A. chapter in the Washington metro area. “It’ll be interesting to see how the Democratic establishment wants to move forward into the midterms.”

Zohran Mamdani, 34, who twice beat Andrew Cuomo, the former New York governor, in his unlikely rise to the New York City mayor’s office, is in many ways the lodestar for the rising brigade of democratic socialist candidates. He unapologetically pledged in his inauguration speech to “replace the frigidity of rugged individualism with the warmth of collectivism.”

He has since moderated positions in deference to the political realities of governing a city of 8 million. He retained Jessica Tisch, a relatively moderate billionaire heiress, as police commissioner and ceded significant policy control to her. He has backed away from his vow to give up unilateral control of the school system, and from his pledge to expand an expensive housing subsidy program.

He has developed a strong working partnership with New York’s governor, Kathy Hochul, a relative moderate in the Democratic Party, and he has struck up a surprisingly amiable relationship with President Trump, despite once characterizing him as a despot. “Sewer socialism,” with images of an army of volunteers shoveling snow or squads of pothole fillers, has become as much a Mamdani calling card as his campaign promise of free buses.

The act of governing is the big test for a movement propelled by idealism and bold promises, along with a disenchantment with the compromises that its followers believe are too often made by those in power.

But fiscal constraints on municipal government can be strict, particularly in Washington, a federal enclave subject to extensive congressional oversight. And at a time when Washington’s finances are suffering from the impacts of federal job cuts as well as a lingering pandemic downturn, the city has had a hard enough time paying for the social programs already in place.

“Especially at the local level, governing is a practical affair,” said Mary Cheh, a former council member who endorsed Ms. Lewis George’s main rival in the primary but acknowledged the appeal of her message.

“There will be some change, I’m sure,” she said. “But it’s not going to be all that they hoped for.”

The limits of idealism have inevitably led to compromise and, at times, friction.

In Los Angeles, Nithya Raman, 44, a City Council member and a democratic socialist, is in a runoff against Karen Bass, the Democratic mayor who is running for re-election. Ms. Raman’s ascent in 2020 coincided with the Black Lives Matter protests that rippled through big cities across the country.

Support for Ms. Raman in her first race that year, against an incumbent on the City Council, became a kind of social shorthand for progressive politics at a moment when flying a Black Lives Matter flag outside of a home was de rigeur among Los Angeles’s wealthy liberals.

But in recent years, Ms. Raman, as a council member, has broken with the D.S.A. on some issues, including how to alleviate Los Angeles’s crushing housing crisis. While she and her D.S.A.-aligned colleagues have both sought protections for poor tenants, Ms. Raman has also backed more development-friendly housing policies.

Up the coast in Seattle, Katie Wilson, a self-identified socialist but not a D.S.A. member, has largely avoided the ideological battles many had expected after her upset victory in November.

Tension between Ms. Wilson and a Seattle City Council that is more moderate has so far led to negotiations rather than conflict, as when she agreed to turn on newly installed security cameras in the city’s stadium district during the World Cup, despite her initial opposition.

Like many of her fellow politicians of the left, Ms. Wilson has made housing a priority. She promised to open 500 new shelter beds or emergency housing units by the start of the World Cup but appears to have fallen short by more than 400. She has pledged to build 1,000 new units by the end of her first year and 4,000 by the end of her four-year term, a tall order.

“I certainly have a learning curve, but I don’t want to portray myself as coming in with some kind of unrealistic idea that this would be easy,” she said in an interview last month. “There’s the way things have been done for a very long time, and it takes a very long time to change that. I’m not surprised at where we’re at.”

But at a time when voters across the political spectrum feel like government has stopped working for them, the promises of a robust and responsive public sector have clearly resonated among voters, regardless of the fiscal or partisan realities.

“When people see you deliver on the small things,” Ms. Lewis George said, “they trust that you can also deliver on the big things.”