In the back yard of a Brooklyn bar, beneath strung-up lightbulbs and swaths of fabric that swooped like great sails, an ecstatic crowd greeted Zohran Mamdani, the New York City mayor, and his victorious ally, Brad Lander. These Democrats also had a withering verdict on their own party establishment.

“To me, centrists can go fuck themselves,” said Léa Zimmerman, 34. “They’re fucking useless, they don’t stand for anything, and if they do stand on something, it’s pathetic. I’m done with pathetic, performative people.”

On Tuesday the biggest city in the US, and the wealthiest city in the world, rejected the status quo in favour of democratic socialism. Five days earlier Washington DC, the nation’s capital, did the same in a mayoral primary election. Los Angeles, the heart of the global entertainment industry, could soon follow suit. Leftwing candidates have also notched up wins in Illinois, Michigan, New Jersey, Pennsylvania, Washington state and Wisconsin.

Once sacred tenets of US politics – unyielding support for Israel, unquestioning faith in capitalism – are under threat from surging frustration with the old way of doing things. No one personifies the tectonic shift better than Mamdani, 34, a democratic socialist who is also the first Muslim mayor in New York’s history.

As results rolled in on Tuesday evening, he flexed his political muscle after backing three insurgent candidates in Democratic primary elections for the US House of Representatives. All had promised to “abolish ICE [Immigration and Customs Enforcement]”, condemned Israel’s “genocide” in Gaza, and vowed to “tax the rich”. All emerged triumphant.

Lander, a progressive, ousted the two-term incumbent congressman Dan Goldman; Darializa Avila Chevalier, a democratic socialist and former campus organiser, toppled Representative Adriano Espaillat, head of the Congressional Hispanic Caucus; and Claire Valdez, another democratic socialist and former union organiser, defeated Antonio Reynoso for an open seat in Brooklyn and Queens.

The trio could be among more than a dozen leftwingers heading to Washington next year as part of what is being dubbed “squad 2.0”, a significant expansion from the original “squad” of four progressive women elected to the House in 2018. That might be enough to wield influence on Hakeem Jeffries, the Democratic House minority leader, by withholding votes on party priorities until their demands are met.

Mamdani enjoyed even more victories in state legislative races, where he successfully backed five other candidates, and wants more. The mayor said he hopes to “write a new chapter in our party’s history, where working people are back at the heart of that struggle”.

Democrats of all stripes were quick to recognise him as an ascendant force within the party, likely to wield influence on the national stage.

Bill Galston, a senior fellow in governance studies at the Brookings Institution thinktank in Washington, said: “Up until Tuesday night, Mamdani might have been dismissed as a fluke – an extraordinarily talented, energetic and personable young candidate, who was able to take advantage of extraordinary weakness in other quarters of the Democratic party to power his way to victory, but that it would be difficult to replicate him.

“As a result of Tuesday night, we have to say that – at least in the bluest parts of blue America – he represents a structural force, and not just a personal talent. I view this result as increasing the already substantial odds that there will be a credible leftwing candidate for president in the 2028 Democratic primary.”

‘Reaping what they sowed’

Like Barack Obama and Donald Trump, Mamdani inspires hope in some, and fear in others. Progressives see in him an overdue course correction, and a vital infusion of energy required to combat the US president’s authoritarianism. Many believe that top Democratic congressional leaders such as Jeffries, a New Yorker who endorsed Espaillat and Goldman, and Senate minority leader Chuck Schumer, have failed to raise their game.

Ezra Levin, cofounder of Indivisible, a grassroots movement behind the nationwide No Kings demonstrations against Trump, said: “Despite our advocacy, despite organising three of the largest protests in American history, what we saw was a Democratic establishment that too often was in freeze mode, or appeasement mode, or in feigning resistance, or engaging in the aesthetics of opposition, while simultaneously engaging in status quo politics.

“The results of that are understandable primary challenges from people who want to see a Democratic party that fights, that isn’t beholden to special interests or to corporate money, and want … an actual opposition party in this country. For Democratic party leadership, this result was entirely avoidable. They’re reaping what they sowed.”

The reaction from Democratic leaders this week was mixed, at best. Some regard Mamdani not as a saviour, but an existential threat. Centrists warn of electoral suicide, fearing that a leftward lurch will doom their chances in swing states in November’s US midterm elections. For Third Way, a centrist Democratic thinktank in the tradition of Bill Clinton, the New York results were a disaster.

Matt Bennett, its executive vice-president of public affairs, said: “It’s a bummer, and it’s a problem – not because there’s any danger of losing those districts to Republicans, but because those kinds of candidates and their ideas and rhetoric can be weaponised against Democrats running in tough races. We’ve seen this before, with ‘Defund the police’, and we’re worried about it happening again.”

The concern was echoed by Joe Walsh, a former Republican congressman turned Democrat and Trump critic. He warned: “Democrats have given Republicans a huge fucking gift. Republicans will do what I used to do: one crazy lefty says ‘defund the police’, and then Republicans and conservative media tag the whole party with ‘defund the police’. They’re gonna try to make the whole party out to be crazy, America-hating, democratic socialists and, to a degree, it’ll work.”

Republicans, facing a hostile climate in November’s midterms, believe Mamdani’s rise has thrown them a lifeline and could give them a unifying cause. Mike Johnson, the House speaker, said: “The Democratic party, the socialists, the Marxists, have nominated some of the most radical candidates to ever run for office, and they’re running for Congress. The insurgent left is on the rise.”

Trump weighed in, as he so often does, on social media on Thursday. “The Communists are finally making their move. I’ve been waiting and preparing for this a long time,” the president wrote. “It’s easy to be a Communist – All you have to do is say, ‘I’ll give you everything,’ but that means you’re taking it away from others that have earned it. Over thousands of years, that Ideology has not worked once. The game is on. Enjoy watching!”

‘Hungry for change’

Another word likely to prove fodder for Republicans is “socialism” – until recently considered a political taboo in the US, conjuring images of Soviet communism and radical Marxism. But a survey by Gallup last year found that Democrats favour socialism over capitalism by 66% to 42%. The gap was widest among voters under 30: the driving force of the Mamdani coalition.

Joseph Geevarghese, executive director of Our Revolution, a group born out of democratic socialist Bernie Sanders’ 2016 presidential campaign, said: “In this particular moment, the label is no longer a scarlet letter. Socialism is no longer an albatross to hang around your opponent’s neck. People are hungry for change, and looking beyond the labels to see what people are willing to fight for – and who they’re willing to fight against.”

Economic populism and affordability are key. The winning candidates were specific in assailing Big Tech, property developers and corporate price gouging. They demanded free childcare, low-cost municipal supermarkets and rent freezes. High rents and gentrification were framed not as inevitable trends, but as the result of government officials serving powerful oligarchs.

The Democratic Socialists of America (DSA), which recently surpassed 100,000 members across chapters in almost all 50 states, has spent years methodically laying the ground for this moment. Ashik Siddique, the group’s co-chair, said: “A big part of our task is diffusing many decades of propaganda from the cold war and red scares in the United States that created a stigma against socialism. We’re helping recover the American roots of socialism.”

Siddique points to figures such as Martin Luther King, who championed economic justice along with civil rights. “We want to emphasise that this is not some foreign thing,” he said. “The Soviet Union is not relevant at this point. It’s been decades since that. Young people especially are not susceptible to the propaganda as much.

“Conditions are challenging for people who are millennials, and younger. There’s not a lot of faith in the existing system. They are much more receptive to the idea that capitalism is causing the problems of our society today, and there’s a better way to organise how we live and work.”

Centrists have failed to identify the source of this American anxiety, according to Cristina Tzintzún Ramirez, a partner at Ascend Strategy Labs, a social justice consulting firm in Austin, Texas. “Where the progressive wing succeeds is that they are very clear about who the villain is,” she said. “A few years ago it was ‘take on Wall Street’; now it’s ‘take on Big Tech’ and how that is squeezing the American people.

“Moderates in this moment feel unimaginative, and unpowerful, because they’re not talking fully about who we’re up against, besides Donald Trump. What we’re up against is economic inequality, and wealth consolidation, and that’s why progressives are winning.”

A crucial litmus test

Young people’s frustrations with the military industrial complex, the hypocrisy of US foreign policy, and the influence of money in politics, has reached a head with US support for Israel’s war in Gaza. In a dramatic shift, about 60% of Americans now view Israel unfavorably, including 80% of Democrats and 57% of Republicans under 50, according to Pew Research.

It is a litmus test that Democrats can no longer avoid. Whereas Joe Biden was dubbed “Genocide Joe” by pro-Palestinian supporters over his support of Benjamin Netanyahu, the Israeli prime minister, Mamdani has accused Israel of committing genocide in Gaza and last month refused to attend New York’s annual Israel Day parade.

Progressive leaders argue that funding from the American Israel Public Affairs Committee (AIPAC) has become “toxic” for many Democratic voters, particularly younger demographics. In the primary between Lander and Goldman, for example – two Jewish politicians running in one of the most heavily Jewish districts in the US – Lander called for an end to US military aid to Israel. He won in a landslide.

Walsh, the former congressman, believes it is now practically impossible for a moderate who firmly supports Israel to win a future Democratic presidential nomination. “I’m very pro-Israel, and I know I’m out of step with where the party is,” he said. “Every time I bring up my support of Israel, Democrats just jump down my throat. It is one of the most powerful animating issues in the party right now.”

Does Walsh, who was inside the rightwing populist Tea Party movement in 2010, see parallels on the Democratic side today? “I’ve been saying for a while the Democratic party needs their Tea Party moment. They’re having it. The Democratic party establishment sucks. It’s about time there’s a Tea Party in the Democratic party.

“The problem is, the Tea Party morphed into [Trump’s Make America Great Again (Maga) movement], and now Maga has taken over the Republican party. If the Democratic party’s Tea Party, these democratic socialists, take over the Democratic party, then we have two parties that are uber extreme. And then we need a brand new party.”

Once the midterms are complete, attention will shift to the race for the White House. Mamdani, born in Uganda, is not eligible to run. But he could again play the role of kingmaker. Speculation is already swirling around progressive figures such as Alexandria Ocasio-Cortez, the representative of New York, and Ro Khanna, representative of California, both of whom have built national profiles.

“As candidates start to announce [their presidential campaigns] in 2027, if not before, you’re going to see very aggressive contests to own the progressive lane,” predicted Geevarghese, of Our Revolution. “People are going to look to Mamdani to try to figure out what is the winning message and strategy. They are going to find his track record very appealing.”

The mayor, a devoted fan of Arsenal football club and the New York Knicks basketball team, is on a winning streak. “When does the race for 2028 begin?” he asked on stage with his slate of candidates last week. “It starts now.”

Additional reporting by Adam Gabbatt.